In the original books, there's this line:
Gains in experience points will be relative; thus an 8th level Magic-User operating on the 5th dungeon level would be awarded 5/8 experience.
(Men & Magic, p. 18)
A lot of people ditch the level multiplier completely. Those that don't usually apply it uniformly to both XP for monsters slain and XP for treasure brought home from the adventure, because that's actually what the rules say.
But I'm thinking "Screw it. Reduce monster XP, keep treasure XP unmodified."
It makes treasure way more important. It's already more important than monster XP (about 80% of all XP earned, some people have calculated.) But I say: Let's go farther.
When a starting adventure kills a bunch of goblins and then tells stories about it in the taverns, it sounds impressive. That's where the XP comes from. But when adventurers keep killing goblins, it becomes less and less impressive each time. XP for repeat kills should count less and less, as the local tavern dwellers are going to say, "Yeah, yeah, yeah, you killed some goblins. We've heard that before."
The easiest way to make repeat kills matter less and less over time is to adjust monster XP based on character level. OK, it might be easier to just keep track of which monsters a character has already slain and not count them again, but that's way more bookkeeping.
I'm thinking maybe a simpler method than multiplying XP by a ratio might be to compare character level to monster level and subtract half the monster XP per level of difference, so that 1st level monsters become worthless when the character is 3rd level, but I'm not sold on that method yet.