So two years ago, when I started compiling my own version of the Classic D&D monsters to be able to print out and use in my games, I decided to use the original OD&D version of XP per hit die, 100 per. For special ability bonuses, I used a straight +50. This is nice and simple, and allows low to mid-level characters to rise in power faster. For me, playing as rarely as I do anymore, that's a good thing. Yes, it makes combat more lucrative, but that's a price I'm willing to pay.
The unfortunate side effect of this is that high level monsters can get devalued. Monsters without special abilities aren't much different after 9 hit dice, but those special ability bonuses get big fast.
So in order to increase the value of powerful monsters with special abilities a bit, I'm considering the following:
Less than 1 hit die: 50xp, bonus 25xp
1 full hit die or more: 100xp per hit die base
1 to 5 hit dice: bonus +50xp
6-10 hit dice: bonus +100xp
11-15 hit dice: bonus +250xp
16+hit dice: bonus +500xp
Bonuses are counted for any asterisk after hit dice or the monster's name (only hit by magic weapons), and for any bonus hit points on top of hit dice. So a hobgoblin, 1+1 hit dice, is worth 150xp.
It's still a lot lower than the normal tables, though. I may increase it further after I think about it a bit more.
I'd rather not have monsters more valuable to fight when PCs are most vulnerable, and less valuable to fight when they actually may stand a chance (or can more easily bring those fallen back).