The great level drain debate usually includes a claim by level-drain haters that it's just too complicated to keep track of, since level affects so many other character elements. Of course, ability score drain also involves extra record-keeping, and in OD&D, the record-keeping for level drain easily beats the record-keeping for six ability scores treated as individual hit point pools. But presumably the level-drain haters are playing later editions, with all their THAC0s and proficiencies/skills and automatic ability score increases and other gewgaws. If you had to erase all that stuff as your levels drop, that would be annoying.
I'd like to tell you "don't play editions like that, then." But I doubt that would go over well.
Instead, my suggestion is to not drop skills, feats and other level-specific abilities at all. Just leave them on the sheet. If the character's ability scores increased, they stay the same. Even for spells memorized, the GM can just note that spells above a certain level aren't available and move on. The number of lower-level spells currently memorized doesn't change, nor do hit points, but once lost, they can't be regained above the normal maximum for the current level.
If you're recording THAC0 or the even more hideous BAB (or whatever the kids call it these days,) that may be a problem. But I'd suggest not using either of those and switching to Target 20 for attacks. Or, use the number of levels lost as a penalty on the roll. Alakazaam! You've eliminated another number to keep track of.
When you regain levels, you don't gain any new skills, feats, ability score improvements, or other gewgaws until you've surpassed your original level.
If you do level drain this way, there's only one thing to keep track of: the level itself. Maybe two things, if you don't trust yourself or the players to remember the highest level reached.