Hard to say without more information (books allowed, etc.) but the obvious ones are:
1) Practiced Spellcaster. Keep your caster level high, at the cost of a feat.
2) Item creation.
3) Metamagic. Particularly Quicken (so you can blow off more of your large quantity of spells) and Empower (to get more bang from those lower level wizard spells).
Alternately, pick a second pclass and set yourself up for it, if you're going to be playing that long. A Theurge tops out at 16th level, when you're equivalent to a 13/13 caster. Archmage is a good bet, if you want to emphasise wizardry, Loremaster and Divine Oracle are good choices (with a good synergy between them) if you want to emphasise clericy.
Finally, you can do things to make up some of what you give away being a theurge. Pick up feats like Great Fortitude, Improved Toughness, or Weapon Focus. Or shore up some of the general weaknesses of spellcasters with Improved Unarmed Strike and Improved Grapple, Clever Wrestling, or Close Combat Fighting.