A blessed ring of sustain ability will protect you from getting level drained, but only once. This could possibly be achieved by removing the blessing, so you could protect against another level drain by re-blessing the ring; or it could be implemented by setting a flag on the ring, so once a ring protects you, it can’t ever be used again; you would need a different ring to get the benefit again.

Further nerf drain-for-gain tactics by having the game remember what you spent each new experience level’s skill slot on and automatically restore those skill slots when you regain levels you lost. If you want to reskill, you can use amnesia.

Vampires shouldn’t be able to harm any monsters with a bite attack if the victim is bloodless, not dealing any damage or level drain. (Also includes the player if polymorphed into a bloodless form.)

When a foocubus drains an experience level from you, it gains a level. (This doesn’t work the other way around because the hero doesn’t have level-draining powers).

Vampire bats that are actually transformed vampires treat their bite attack as if it drains life (as with a regular vampire’s bite) rather than dealing physical damage.

Amulet of drain resistance, which also gives immunity to death magic. Useful if monsters casting drain life are implemented.

Vampires drain levels much less than they currently do, or not at all; instead they use a bite with an AD_DRHP damage type (described in #1627). Possibly give vampires one AD_DRHP attack but let vampire lords keep one AD_DRHP and one AD_DRLI.

Foocubi gain a level when you get a negative result from them. This should perhaps be extended to other types of enemies with draining powers.