Class
Since the question is about how to improve the marshal, makes the most sense to start with that, and other classes that can complement and synergize with the marshal.
Marshal (Miniatures Handbook)
The marshal has very, very limited opportunity to optimize. They only have four class features:
Skill Focus (Diplomacy)
This is just a feat, and a fairly basic one at that. There are loads of things you can do to improve Diplomacy, and taking Skill Focus would be a step most diplomancers will likely take, so hey, you get it free. But it’s not really “special.” You optimize it by optimizing Diplomacy.
Note that Diplomacy is a really problematic skill. As written, high Diplomacy checks can allow you to just tell people what to do, even if they should hate you. Hence the tongue-in-cheek “diplomancer” to refer to characters who have very high bonuses. No one runs it like that, or should—but as a result, it becomes very unclear what a high Diplomacy check should get you.
At any rate, optimizing a diplomancer has little to do with the marshal class—the only relation is that marshal could be a source of a feat that a diplomancer probably wants. Look up any of the many Diplomacy/diplomancy handbooks out there for more on how to get the most out of the skill; it isn’t hard to break the game, as written.
Super-niche, but there is another application of this class feature that actually is unique to the marshal, not that I’ve ever found a particularly good use for it—if you already have Skill Focus (Diplomacy) when you take marshal 1st, you get to choose another feat you qualify for. This is actually very rare, officially, though many if not most DMs just allow it for all bonus feats. Anyway, you can leverage this if for some reason you want to “delay” a feat you otherwise could have chosen until later, presumably because the feat you want requires things you don’t have yet. You can take Skill Focus (Diplomacy) instead, and then take your 1st level of marshal when you want to get the other feat.
Minor aura
This is probably the best class feature that the marshal has. It’s certainly the one most amenable to optimization, which is to say it can be optimized, at all, because none of the others really can.
The idea with your minor aura is to pump your Charisma as high as possible, so that when you, say, motivate Dexterity, you’re offering a large bonus.
Dexterity is, by the way, probably the best target: since initiative is a Dexterity check, motivate Dexterity applies to it, and giving +Cha to your entire team’s initiative checks is amazing. Initiative is often the most important roll in a battle, so that’s nothing to sneeze at.
Master of tactics and the various save boosters aren’t bad, either—sadly, you can’t ever have more than one minor aura active at a time.
Since optimizing this means pumping Charisma as hard as you can, it is very, very worthwhile to look for other opportunities to accomplish “Cha SAD-ness,” where “SAD” means “single attribute dependency.” In other words, since you are getting a stratospheric Charisma score, you want to apply that score to as many things as you possibly can. Unfortunately, minor aura is the only marshal feature that accomplishes that, and can only do it for one thing at a time.
Major aura
Ironically, this isn’t as good as the minor aura. Charisma can be huge; this bonus starts at +1 and grows extremely slowly.
The save bonus is almost-certainly the best target for it. The DR isn’t bad—particularly at low levels—but ultimately the saves are more important.
You can’t have more than one major aura active at a time.
There are a few feats that improve auras, but they’re basically uniformly garbage. So the optimal thing to do here is basically not do anything, because putting resources into this feature is largely wasted.
Major aura also qualifies you for the Draconic Aura feat from Dragon Magic, allowing you to take dragon shaman auras. Those are mostly meh, too, but the “fast healing up to half health” is decent enough, as are the fire and cold resistance ones. Again, since you can’t have multiple auras at a time, it’s really hard to leverage this, though.
Grant move action
This is great!
But you only get it a very few times per day.
There isn’t really any way to “optimize” this, save for using it on the allies who will get the most use from it. That would usually be anyone who wants to full-attack, but doesn’t have anyone in reach.
So most of these can’t really be optimized in any meaningful way. The ones that can—kinda—are basically potential building blocks for optimizing other things (Diplomacy, Cha SADness), but aren’t things the marshal is particularly good at alone. More importantly, you get all of these abilities by 4th level—and you get the best, most optimizable one, at 1st. More levels let you learn more auras—but you can still only have one minor and one major aura at a time, which really limits the value of learning your second- or third-favorite aura of each type. Major aura also grows in size, but extremely slowly—taking 5–7 levels just to get a +1 bonus is not an optimal strategy by any means. More uses of grant move action would be nice, but at the rate you get them, it’s too much.
All told, it’s really hard to make marshal more than 1–4 level dip. Maybe 5th, just because you get a 3rd minor and a 2nd major aura, though again those are going to be also-rans against what you already have. With your party—which is heavily magical and not looking for full-attacks—even 4th level seems hard to justify. If one of those casters is playing a summoner, maybe?
And to put marshal to the best use, we need other class features—because we want to go for Charisma for everything.
Bard (Player’s Handbook)
An obvious choice, bards’ music works a lot like marshals’ auras, and fill a similar niche. Note that bardic music doesn’t actually require music per se, just an audible performance, so no worries about having to turn into a musician—Perform (oratory) should be just fine. Unlike auras, bardic music is very, very amenable to optimization—inspirational boost (Spell Compendium), Song of Creation (Eberron Campaign Setting), and/or medallion of courage (Magic Item Compendium) can easily get your inspire courage up to +3 even as a 1st-level bard. Words of Creation (Book of Exalted Deeds) can double that, though honestly I’d suggest that may be going too far.
Dragonfire Inspiration (Dragon Magic) grants a new song that adds Xd6 fire damage to allies’ attacks, where X is the bonus from inspire courage—meaning all of the above optimization applies to that as well. Since it’s a separate song, you can overlap the two and get +X attack and +Xd6+X damage, which is rather impressive. (Some DMs may decide that the two are two closely related to stack them like that, though.)
Song of the White Raven (Tome of Battle) lets you start inspire courage as a swift action. This is amazing, though qualifying as a bard/marshal is hard—more on that in a bit.
Snowflake Wardance (Frostburn), though, is the real reason we are here. This feat gives us a new usage of bardic music that allows us to add Charisma to attack rolls with slashing weapons used in one hand. This is great for us, because it allows us to use our Charisma for one of the big important things we want to do.
Paladin (Player’s Handbook)
You want to focus on Charisma? Then if you don’t have divine grace, you have made a mistake somewhere. Divine grace is arguably the best non-scaling class feature in the game. It is preposterous how strong divine grace can make your defenses. A Charisma-heavy paladin can actually make an excellent off-trapmonkey by just walking through the traps and almost-certainly making all the saves.
Conveniently, there is a variant paladin, the harmonious knight (Champions of Valor), that gets inspire courage +1, 1/day at 1st level, instead of at-will detect evil. Not saying that detect evil isn’t nice, but considering everything we just said above about inspire courage, I hope it’s clear that inspire courage is a whole lot better. Note that to be a harmonious knight, you need to worship Milil—at least, you do in the Forgotten Realms. As Champions of Valor is a Realms-specific book, in other settings your DM may adapt it to another god—or just ban it entirely. Ask them about that. As a harmonious knight, you don’t actually need to take bard levels at all—though if you don’t, you’ll probably want to take Extra Music (Complete Adventurer)—each one gives you 4 additional uses per day, so even taking it once should just about do you, unless you really want to layer inspire courage and Dragonfire Inspiration and Snowflake Wardance on every fight.
Paladin is a 2-level class. We get divine grace and get out.
any class that can get Charisma to AC (various)
As long as we’re getting Charisma to our saves, how about that other defense, AC? Monk, obviously, has Wisdom to AC while unarmored—you can make that Charisma using Ascetic Mage (Complete Adventurer). But monk isn’t the only class to get Wisdom to AC, and Ascetic Mage works on any of them. You could also do “swift and deadly hunter” druid, moon-warded ranger (Dragon vol. 340), or swordsage (Tome of Battle) are all options as well—and the ranger or swordsage get to use light armor and still get the bonus (!), though they each take two levels to get it.
If you don’t want to spend the feat, or can’t qualify (e.g. didn’t take bard levels), battledancer (Dragon Compendium) has Charisma to AC while unarmored without taking any feat. Battledancer is a lot worse than monk (and that’s saying something), but depending on your build getting it cheap might be better than jumping through hoops.
Crusader (Tome of Battle)
And here we are, my real answer to this question: everything the marshal was trying to do, the crusader does better. The White Raven discipline is all about battlefield leadership, and it’s excellent. Where the marshal is giving out a move action a few times per day, a crusader can give out an entire turn once per fight (White Raven tactics).
I see from comments that some members of the party are “iffy” on Tome of Battle. They shouldn’t be—it’s easily the best-designed book in all of D&D 3.5e, and its classes are extremely tightly balanced with one another and fall squarely in the lower-middle range of 3.5e classes’ power levels. Each of the other members of the party is playing a class potentially much stronger than anything available in Tome of Battle.
Furthermore, our use of the book is going to be a lot of support and utility anyway. You shouldn’t be stealing the spotlight—you should be aiming the spotlight at your allies.
Now, White Raven is better with martial allies, there’s no doubt about that—but that’s true of the marshal, too. But nothing locks you in to White Raven as a crusader—you also get Devoted Spirit and Stone Dragon to play with, and both are great. Those will go a long way towards improving your battlefield presence, whether it’s through buffing or your own actions. Focus on White Raven—it’s on-brand, it’s better for your party’s concerns, and it’s strong—but grabbing stuff like Devoted Spirit’s healing maneuvers, or the mountain hammer line from Stone Dragon, can give you a ton of extra utility. (Note that mountain hammer ignores hardness, making it great for smashing through doors—or walls.)
And best of all, it qualifies you for Song of the White Raven, and with that feat, your crusader levels stack with bard for determining how strong your inspire courage is (not, sadly, how many times per day you can use it, so you’ll still want Extra Music).
It is generally advantageous to take your first crusader level later, since your non-crusader levels apply half to your crusader initiator level (this is also true, by the way, for the swordsage that we discussed for AC). Which works out nicely, since there are some non-crusader levels we want to take.
Race
Human. You want a bunch of feats, and human gets you an extra one. Moreover, the only ability score you really care about is Charisma—and racial Charisma bonuses are rare and almost-always come with a Constitution penalty, which is bad. If you want to be a venerable Dragonwrought kobold (Races of the Dragon), or a lesser aasimar (Player’s Guide to Faerûn), those races would also certainly be powerful for you, but those are too cheesy for most games.
Notably, there is a human subrace known as silverbrow humans (Dragon Magic), which drop the extra skill point in favor of the dragonblood subtype (and some other minor things). This is nice just because dragonblood is required for Dragonfire Inspiration—not your top feat priority, but certainly a feat you’d very much like to have eventually, and having to take another feat—Dragontouched—to qualify is painful. I suppose you could dip dragon shaman (Player’s Handbook II) or dragonfire adept (Dragon Magic) to get more auras while you meet that qualification, but I don’t consider either worth it.
Conclusion
I strongly recommend that you go with a mishmash here, because you want several things available in the first couple of levels of several classes, and they synergize extremely well to accomplish the kind of martial buffing you’re interested in.
A key, but controversial, component here, I think, is having your 1st level be bard. Bard has the advantage of 6+Int skills—so you get the most out of quadrupling them for your 1st level—and qualifies you to take the 1st-level-only Precocious Apprentice feat (Complete Arcane pg. 181, not with the rest of the feats). That gets you a 2nd-level spell, which, since you’re a bard, means you meet Ascetic Mage’s requirement of being able to cast a 2nd-level arcane spell spontaneously (nothing in the feat says it counts as prepared). Then two levels of ranger or swordsage get us Charisma to AC in light armor, which is awesome.
I favor swordsage because maneuvers are great, but feel free to go with ranger if you prefer the BAB. Note that, like the crusader, it’s best to delay swordsage levels, so even though we’re taking bard 1st in order to be prepared to take Ascetic Mage, it’s probably better to hold off on that until about 6th level.
If you can’t do bard-first Precocious Apprentice, or don’t want to, Ascetic Mage’s 2nd-level spell requirement looks too painful for us, so we should probably skip it. Charisma to AC means battledancer, or we just ignore that and just wear armor—honestly, it should be fine. Ranger or swordsage are probably too far out of our way now, so battledancer can come in whenever if we want it.
Either way, after the one level of bard, your next priorities are marshal 2nd or 3rd, and paladin 2nd. If you can be a harmonious knight, and aren’t doing Ascetic Mage, you can even skip bard altogether in favor of just going straight to marshal and paladin—your skill points suffer, but a level is a huge thing. This also makes it more reasonable to take marshal out as far as 5th if you really want to, though again, grant move action is a lot worse than White Raven tactics, particularly for your magic-heavy party, and it’s somewhat doubtful that your second-favorite major aura will be that valuable.
And after that, just go all-in on crusader.
The two approaches work out to something like one of these:
2nd-level harmonious knight/5th-level marshal/1st-level battledancer/11th-level crusader
You want to start with harmonious knight paladin just to qualify for Snowflake Wardance, Extra Music, and Dragonfire Inspiration for your human bonus feat, 1st-level feat, and 3rd-level feat. Then you can take Song of the White Raven at 9th. Your 6th-level feat is probably best off as Song of the Heart, but whatever works.
Note that you should feel free to tune your number of marshal levels to your preferences—you should weigh carefully whether or not your second-favorite or third-favorite aura is worth delaying crusader. This build has the advantage of taking crusader at 9th when you get 3rd-level maneuvers and stances from your initial set, but getting maneuvers at all a level or three sooner is worth more than that.
If you can’t be a harmonious knight, add a level of bard instead of a marshal level or a crusader level.
1st-level bard/3rd-level marshal/2nd-level paladin/2nd-level swordsage/11th-level crusader.
Either your 1st-level feat or your human bonus feat has to be Precocious Apprentice for this to work.
You probably want to be a 1st-level bard/2nd-level marshal/2nd-level paladin/1st-level swordsage at 6th, because you’ll want to take Ascetic Mage at that point and swordsage is how you’re getting Improved Unarmed Strike (see the unarmed adaptation on page 20, and note that while it costs you proficiency in light armor, it doesn’t change how your AC bonus works so it still works in light armor). Level-up marshal and swordsage again after that before going into crusader at 9th.
Aside from that, feats are the same as for the above build. Song of the Heart and Dragonfire Inspiration will necessarily be delayed until after Song of the White Raven, so probably 12th and 15th, respectively.
Both of these builds want as much Charisma as humanly possible, because they add Charisma to attack, AC, and all saves, as well as to the initiatives of everyone in their party. Both allow for powerful buffing in the form of auras and bardic music, get decent skill point totals, and allow you to be a respectable combatant in your own right (at least once crusader comes online). The former build is simpler, has more of the marshal class, and allows you to switch that Charisma bonus to the party’s initiative to more things. It also has more feats to play with at higher levels. The second build can wear light armor, has more maneuvers, and therefore should have more of a combat presence, particularly in the 6th–8th level range.
Neither build is going to keep up with mid-to-high-level spells. If the rest of your party plays those classes to the hilt, you will be outclassed in the back half of the game, and the most meaningful thing you’ll be able to do is make sure everyone on your side gets to go first with motivate Dexterity, and then White Raven tactics to give one of the spellcasters another turn.
If you are playing with multiclass XP penalties, you should stop doing that, because they’re terrible rules—but if you can’t, you’re basically limited to marshal 2nd in either build. Taking crusader earlier and for more levels is the obvious adaptation, but a there are other dips you could take instead of those levels that could also work well. I’m not going to bother delving into those unless requested, though, because almost no one plays with those penalties and no one should.