Depends on what idea I'm going for, but generally speaking.
If I'm doing the diviner I'll usually give up necromancy. Sure you lose out on Ray of Enfeeblement, Blindness/Deafness, Spectral Hand, Enervation, etc., but if you're making information your business you don't really want to give up enchantment or illusion as they compliment divination's ability to gather intel nicely.
If I'm another kind of specialist, I'll usually give up two out of either necromancy, illusion, or enchantment. Such as if I'm an evoker or conjurer, your real direct reigns-o'-power battlemage type, I'll give up enchantment and illusion because they're 'soft' disciplines. That sort of idea. Stay within concept for what schools go. *nod*