Avoid those situations.
Of course, that's not always an option. As a wizard, a simple Invisibility (preferably the Improved or Greater version) might help you avoid getting targeted for long enough to take out the sources of save-or-die effects. Alternately, appearing to be a smaller threat than the rest of the party will cause your buddies to get targeted, and not you.
If you want to flat-out resist them, your big priority is boosting your saving throws, especially Will and Fort. Having high saves is generally a good thing anyway, so investing in a Cloak of Resistance is money well spent. If you're headed for Incantatrix, Iron Will is a prerequisite, and +2 to Will saves is cool. Items that boost your saving throw abilities (dex, con, and wis) shouldn't be too expensive, and help out with other stuff, too. If you don't want to spend a bunch of permanant items, Cat's Grace, Bear's Endurance, and Owl's Wisdom work too.
A good catch-all defense against death effects is Antimagic Field. Sure, you're useless when you put it up, but if you're fighting primilarily magic users, they're just as useless, and might waste a few spells on you, too. Spell Turning has a good chance of protecting you from at least one death effect, if that's what your foes throw at you first. At real high levels, your best friends are Protection from Spells, Prismatic Sphere, and, above all, the uber Moment of Prescience.
Finally, if you've got a cleric who can cast Persistant spells, Spell Immunity, Greater Spell Immunity, and Spell Resistance all are great to have in general. If you don't, spell resistance items are nice, though most aren't cost-effective for the protection you get from casters of your level or above.