1. Anything boosting you or an ally.
...sure, I could list some spells, but really, whatever you or your allies need to be available, Extend/Persist is your friend. Spells that last 10 minutes/level or more are more likely to save you a combat cast, whereas spells that last 24 hours will actually save you a slot every other day. Tyche's Touch, for instance will give you a bonus on Saves for 24 hours - or 48, if Extended.
If you are preparing for an upcoming battle, pretty much any spell of a rounds/level duration or more would be a good way to prepare for it. (Mass) Lesser Vigor would keep all affected in combat longer, auto-stabilize when needed and possibly get them back on their feet.
Shout-out: Creeping Cold. Druid-2, but get and Extend this with the Lord of the Uttercold feat, and things will die. Get Ocular Spell and Persist it, and...let's not talk about that.
2. Multiple sources of Turn/Rebuke, Extra Turning and Nightsticks.
The feat Extra Turning is interesting:
Benefit: Each time you take this feat, you can use your ability to turn or rebuke creatures four more times per day than normal.
If you have the ability to turn or rebuke more than one kind of creature each of your turning or rebuking abilities gains four additional uses per day.
Special: You can gain Extra Turning multiple times. Its effects stack. Each time you take the feat, you can use each of your turning or rebuking abilities four additional times per day.
So, if you add e.g. Rebuke Undead to your character, and get Turn Undead, each feat spent on Turn Undead will get you four Rebukes and four Turns.
It's sort of hard not to lose at least one level of spellcasting. I'd recommend a Dread Necromancer dip for the Rebuking. If you are Good, a Death Delver dip may be your best option - and Deathsense is really good, so it might be your first option, as well.
Nightsticks (here be dragons)
The Nightstick is generally considered quite controversial:
This black rod carved of darkly stained wood
is inset with religious symbols of various deities. Anyone who
possesses the rod and is able to turn or rebuke undead gains
four more uses of the ability per day.
- "Possesses"? Is this slotless? Does it even need to be carried, or brought along on adventures?
- Does it boost both Turn and Rebuke Undead, as Extra Turning does? This sounds plausible, as it lists Extra Turning as a prerequisite for crafting.
- What happens if you have more than one? Does each give you four extra?
- What happens if you drop the nightstick, once you have use up all "charges", and pick up a new one?
However, worst case is you can only have one, and it gives you four more Turns for DMM. Well, as long as the DM doesn't ban the item outright.
Bonus: Easy Metamagic and Practical Metamagic, PrCs
Each DMM'ed Persist can set you back six Turn/Rebuke attempts. However, the feats Easy Metamagic and Practical Metamagic can bring that down by one each, to four attempts.
Prestige Class rebate
There are, likewise, a few Prestige Classes which lowers the cost of metamagic. Notably the Dweomerkeeper, which that previously mentioned dip of Dread Necromancer will have set you up for. At level 10, it gives you Cloak of Mysteries:
At 10th level, a dweomerkeeper is wrapped in a mantle of ever-flowing magic, and all metamagic feats that she currently knows or learns in the future become easier to use. The spell level increase for applying a metamagic feat to any spell drops by 1 (minimum +1 level, or +0 level if the feat already has a +0 level adjustment).
Dweomerkeeper is generally considered a very powerful choice, since it can also give you free Miracles and True Resurrections (or other spells with XP or material costs), so ten levels are probably not wasted.
3rd party shenanigans
In a book I...hesitate to name...exists a three-level Prestige Class called the Metaphysical Spellshaper. It's really easy to qualify for (you will do so once you get Persist, with some skill ranks) and gives you the same rebate as the 10th level of Dweomerkeeper at level 3.
Oh, and a bonus Metamagic feat on level 2, which you can use for Easy Metamagic (Persist).
Level 1 of the class only provides you with another way to pay for metamagic, using Ability damage; I'm not saying that you first should Persist Sheltered Vitality using DMM and then use this all day long, but, arguably, you COULD...