It also depends on the source of the energy drain. The 9th level spell "Energy Drain" is straight-up permanent level drain, and needs to be cured through Restoration, though I'm not sure any enemies actually cast that spell. You can also usually judge by the icon: the version you get from Shadows and Spirit Trolls is a red X and is temporary attribute loss, the red-yellow V-shape is permanent level loss.
Really though, the best way is prevention. Restoration spells cause a lot of fatigue, and buying a restoration spell from a temple will fatigue EVERYONE, not just the person who was drained. I think that (Greater) Restoration will also fatigue everyone, but only if you have ToB installed (and it's a bug, it's only supposed to fatigue the caster). It also doesn't help if you cast a Restoration spell and then another vampire shows up and drains them again.
The best options for prevention are the Amulet of Power, which is available in Chapter 2, and the Mace of Disruption +2, which is available in Chapter 3 (extra shenanigans with the Temple quest required). However, you can only get them if you side with the Shadow Thieves. If you side with Bodhi, you're pretty much screwed in that department. (She claims that you'll get some magical trinkets as compensation for your gold, but you don't actually get anything.) There are a couple of weapons in ToB that have NPP, but by then there aren't many creatures that actually cause level drain, and Turn Undead will basically automatically destroy everything.
You can also use Protection from Undead scrolls, but beware! If you use them against any enemy that's supposed to talk to you (like in Bodhi's Lair or the Temple of Amaunator) you can get some really irritating glitches unless you have at least one character unprotected.