Ultimate Magus (Complete Mage and also online here) can advance both Wizard and Assassin spellcasting, as it requires both Arcane spellcasting prepared from a spellbook, à la Wizard (or Wu Jen), and spontaneous Arcane spellcasting, which the Assassin has by virtue of the fact that “An assassin casts spells just as a bard does.”
You need at least 2nd-level spells on the prepared side and at least 1st-level spells on the spontaneous, so a Wizard 3/Assassin 1 has the requisite spells (though you'll need more levels because of skill requirements of both Assassin and Ultimate Magus).
The Ultimate Magus advances both classes on 7/10 levels, and only one class on the other three levels (at 1st, 4th, and 7th). In each case, the class that is advanced is the one with the lower Caster Level. Since Wizard spellcasting is better than Assassin spellcasting, it is probably best to try to boost your Assassin caster level above your Wizard caster level, so you can benefit from full spellcasting progression for your Wizard side. Practiced Spellcaster (Complete Arcane) helps a lot with this, but may not be enough depending on how you enter the class.
Ideal Case: Wizard 5/Assassin 1/Ultimate Magus 10
You want to enter the prestige class ASAP, you want to do it with minimal loss of Wizard spellcasting, and you want to advance Wizard spellcasting at every Ultimate Magus level, particularly since you’ll already have the highest-level Assassin spells even if you miss three like that.
First, you’ll need Hide and Move Silently as class skills, at least for your Wizard levels and ideally for the Ultimate Magus levels as well. You may also want Disguise in-class, but it’s not a big deal since it caps at 4 ranks required, which you can do cross-class if necessary.
Move Silently is annoyingly difficult; you could be a Viletooth Lizardfolk (Dragon Magic) but with LA +1 you might as well just dip something. Alternatively, an Elf (preferably Gray for the Intellignce bonus) could take Aereni Focus (Move Silently) from Player’s Guide to Eberron to get it always in-class as well as a +3 bonus to it (also counts as Skill Focus if that matters to you).
Hide has two easy options, at least: Unearthed Arcana Wizard variants can get it for you at level one, and the low cost of your Scribe Scroll feat. You could either take a Fighter feat, and use it for Martial Study (any Shadow Hand maneuver), or if you are an Illusionist, you could take the Shadow Shaper ACF to just get it directly (and add your Int to Hide at 5th level, which is pretty cool). I prefer the Martial Study route as it sets you up for Assassin’s Stance and also gets Hide in-class for Ultimate Magus levels. Martial Study and Stance are from Tome of Battle.
I will note that I find the lack of a feat that grants both skills in-class kind of strange; they’re certainly thematically linked and plenty of feats do that sort of thing. An Apprentice feat could reasonably combine the two (Apprentice [ninja] maybe? Hell, Apprentice [assassin]), but none of the examples from Player’s Handbook II actually does so.
If you want Disguise in-class, City Slicker from Races of Destiny is a pretty easy choice. Silverbrow Humans (Dragon Magic) also get it, but I can’t find any way for them to get Move Silently so that’s a bust.
Anyway, have 8 ranks in Hide, Move Silently, and Spellcraft by Wizard 5, along with the 4 ranks you need in Disguise and Knowledge (arcana). This requires at least 5 skills points per level (4 if you have Disguise in-class), but even with 2+Int skill points a Wizard should have at least 6, particularly a Gray Elf who gets an Int boost. Which is good because of course you also want Concentration.
Then at 6th level, take Assassin 1 and Practiced Spellcaster [assassin] from Complete Arcane to boost your Assassin caster level to 5, matching your Wizard caster level. At 7th, it’s Ultimate Magus 1, and your caster levels between Wizard and Assassin are equal, so you can advance Wizard. At this point, you have a choice. You could leave well enough alone, accept that you’ll miss a second Wizard level at Ultimate Magus 4, and thereby gain 9th-level Assassin spellcasting instead of 8th-level.
Or you could attempt to find another CL boost for Assassin before you hit Ultimate Magus 4, and a second before Ultimate Magus 7. It has to be general and permanent enough to qualify Assassin as no longer having the “lowest caster level,” which is difficult, and it has to also not apply to your Wizard caster level, which is harder still. An Orange Ioun Stone probably affects both classes, as does a Bead of Karma. Boosting a single school of magic sounds promising since you can boost one that the Wizard has banned, but probably is not general enough for the Assassin to claim generally higher CL.
In short, I have not yet found a way to do this.
So I’m going to leave it at a Gray Elf Wizard 5/Assassin 1/Ultimate Magus 10, with 14th-level Wizard spellcasting and 9th-level Assassin spellcasting.