Some form of Deathward is pretty mandatory to me. I, personally, am fond of the Soulfire enchantment for armor from BoED. No wizard worth his salt will be taking negative levels.

Countermeasures to dispel are also a must for any caster. From the old core Ring of Counterspelling from the CMage's Ring of Enduring Arcana (+4CL for dispelling purposes), they are important.

And no one can go wrong with CL increasing itens. Ring of Arcane Might, Ioun Stone, Create Magic Tatoo. I'd go further by saying that itens that improve central class features are usually 'must have'. Most warlocks tout a Warlock Sceptre around, for instance.