Thanks, but I can't take credit for it - I've seen it several times.
A Medusa can qualify for Assassin by rearranging some skill points, and Duelist has pretty easy requirements too. Don't be afraid to rearrange some of the monster's skills and feats. Sorry, I can't recall any more immediately, but the easier the requirements are, the more likely that it is that a creature can take it with minimal requirements. Some of the Complete Adventurer PrCs should be pretty easy for intelligent monsters like Mind Flayers.
Here's another couple:
Divergant Monsters - Give your monsters a little twist by swapping out some of their skills and feats for different selections. Or go even farther and swap out some major abilities. An Umber Hulk could trade off it's confusion ability for some energy resistance without drastically altering its CR.
Unusual Tactics - Have the monsters use tactics that the PCs don't normally expect from them. Ogres who usually rush screaming into melee might have some military training: they could form ranks with longspears in front and longbows (ouch!) in back. A mind flayer with a potion of water breathing might attack from an underground stream, or flying kobolds could drop rocks (or incendaries) on the pcs.
