ToME: the Tales of Maj'Eyal

In the late game the thing that wrecks me the most is getting my sustain abilities dispelled. How can I prevent that? A higher spell save?

I think the kind of save used is specific to the effect, so I don't know. For example some spells are resisted with physical save (and may cause an "off-balanced" rather than "spellshocked" cross-tier effect).

Some dispels are affected by saves:
Corrupted Negation (compares against spell save for each sustain/effect to be dispelled)
Spell Feedback (compares against mind save when applied)
Destroy Magic (I'm not sure of the mechanics behind this one)

Some are not:
Manaclash & other manaburn damage (indirectly disables sustains by draining your resource pool)
Disperse Magic

The following status effects can all inflict you with "spell failure," which may cause you to lose your sustains:

• Spell Feedback is a mental status effect caused by using magic against certain powerful antimagic enemies (such as Champions of Urh'Rok).
• Spell Disruption is a physical status effect that, as far as I know, can only be inflicted by the Spellhunt Remnants. You are extremely unlikely to suffer from this, as it would require that a rare Brawler be randomly generated with the Remnants equipped; the odds of that happening are almost negligible.
• Broken Dream is a mental status effect that certain rare Solipsists can inflict on you. It requires that the enemy hold still for several turns, so NPCs aren't usually smart enough to use it against you.

If you have one of the above status effects (and Spell Feedback is the only one you really need to watch out for), you have a chance each turn of losing your magical sustains. The effect description will list an x% chance of failure for each spell cast; each sustain has an (x/10)% chance of being disabled every time you act. Cure these ASAP!

There are also a handful of effects that can turn off your sustains directly:

• The Taint of the Devourer--Cannot be resisted, though fortunately only the Weirdling Beast has this ability. When you confront that monster, make sure you're ready to fight without any sustains!
• Corrupted Negation--Can be resisted with spell save. Found on Corruptors.
• Weapon/Ammunition of Purging--Cannot be resisted, though it only has a 25% chance of removing a single effect or sustain per hit. Antimagic lesser ego.

Lastly, you will lose all your mana-based sustains if you run out of mana (say, due to Mana Clash).