Go for the standard stuff. Quicken Spell is always useful at higher levels, and Extend Spell might be good to have.
I think many of the useful illusions require SR checks, and Arcane Mastery (Complete Arcane) + Spell Penetration and maybe Greater Spell Penetration means you will almost always auto-succeed SR checks. As a bonus, if you have some way of boosting your Dispel checks, you'll almost always auto-succeed on those as well.
Stuff that improves spell DCs also help. Spell Focus and Greater Spell Focus is useful. If you've got lots of feats and have access to Frostburn, then Snowcasting + Eschew Material Components + Cold Focus + Greater Cold Focus grants you +2 DC to all your spells. If you can also use Dragon Magic, there's a more efficient combination of feats: Snowcasting + Eschew Material Components + Draconic Aura (Energy (Cold)) + Dragontouched, which grants you a scaling DC boost to all your spells (+2 at level 7, +3 at level 13, +4 at level 20), and you can tack on (Greater) Cold Focus for more.
If you use a lot of spells that require concentration, Extraordinary Concentration from Complete Adventurer could be useful. Steady Concentration from Races of Stone can help you make those high DC checks.