I'm playing a mystic theurge right now (just hit 5th level in the class, but he has a Smoking Eye, so he's 12th level total. 3/3/5(+1 ECL.)) Couple of suggestions:
All Charisma, All Spontaneous sounds good in theory, but if you can swing the two high stats, go with Wisdom and Intelligence. The theurge's saves suck, except for Will, and that one will suck as well without a pump from Wisdom. Similarly, the theurge has only 2 skill points per level, but a great skill list, so Intelligence is nice for the bonus points. What I'm saying is, you get more from Wisdom and Intelligence than just save DCs, and the theurge needs what you get.
If your DM will go for it (and most should, as it fits the class perfectly), use the cloistered cleric from Unearthed Arcana. It gives you access to another domain (Knowledge), more skills and skill points, and some alternate spells (e.g., identify) which will save you room in your spellbook. The cloistered cleric's weaknesses compared to the cleric (HPs and BAB) are things the theurge can never be better than mediocre at; as is usual in D&D, you'll be more effective if you emphasize what you're good at, even at the expense of making yourself worse at what you're bad at.
Depending on how your DM handles familiars, I wouldn't bother with one. (You might even see about swapping out the ability; again, see Unearthed Arcana.) It will have horrible HPs, and will never progress into the really cool familiar abilities. The exception is if your DM is pretty much "live and let live" with familiars, or actually generous in how he plays them, in which case they're as useful as always.
If you decide to specialize as a wizard, remember that you can use your divine spells to cover some of the bases in your prohibited schools. Similarly, as with the identify example, above, you're going to be able to save spellbook choices by preparing spells that are available to both classes as a cleric. There are also spells -- e.g., blindness/deafness, bestow curse -- where you'll get them at an earlier level through one class than through the other. It's not always advantageous to use the lower-level side, but it's close to always.
Look for spells that have extreme cross-spellcasting usefulness. An obvious example is spectral hand; it's a second-level wizard spell that will last an entire combat, and that, in addition to the usual wizardly uses, will let you cast cures, inflicts, or bestow curses without exposing yourself to nasty melee.
I'll second the utility of Practiced Spellcaster. I'll even go one further and suggest you petition your DM to rename it "Versatile Spellcaster" and allow the +4 caster level bonus to be split. If not, you'll probably have to bite the bullet and take Practiced Spellcaster twice and eat the wasted +1/+1 of caster levels. You'll also want Spell Penetration by the time you're 12th level. Finally, I strongly suggest picking up Craft Wondrous Items as soon as possible. You'll be able to at least partially compensate for many of the theurge's weaknesses by cranking out some homemade magic.
In conclusion, I've really had a great time with my theurge, even to the extent that when he was bludgeoned to death by holy literature (don't ask, I'm still bitter), I agreed to have him raised, and I hate the Respawn Philosophy of D&D; I nearly always make my dead PCs stay dead. Just be careful and play smart, smarter than with any other character type in the game, and you'll really enjoy the seemingly endless spells. You'll never be a walking Howitzer, but you'll also never lack for something useful to contribute in an encounter.