Just a quick idea that I'm not sure I'll ever actually use but thought I could share. Since there are powerful monsters (such as undead) with Level Drain or Ability Drain, how about XP Drain? Level Drain is basically that, so maybe there could be monsters that would Drain a limited amount of XP instead of whole levels right away?
For example, a 5 HD Undead who Drains 1d30 XP with each successful attack.
For all I know someone has already thought of that, I haven't checked.
EDIT: The current version of Death Frost Doom does exactly that, except in huge amounts of XP. So somebody did think of that before me, alas. :p