[Mourn Yauntyyr]

Yeah I agree Negative Plane Protection is way too short. Here's what I did to counteract the affects of level drain:
1) Only non-spellcasters go into melee combat. That way when they do lose levels, they don't lose spells.
2) Those fighters must have very low armor classes. If they can't hit ya, they can't level drain ya.
3)After the fight is over, use lesser restoration (a 4th level cleric spell) to fix the fighters, then heal them back up to their normal hit points.
4) There is an amulet that has a lot of different powers (I forget what it's called). One of those powers is permanent Negative Plane Protection, put that on one of those fighters.

This method worked very well for me. Sure I got level drained but it was never anything catastrophic that required me to rest immediately.