I spotted a fun blog post by Hannah Heath, young aspiring author, on 7 cliched characters in YA fiction that she says need to stop:
1. The character that is full of teen angst
2. The girl that's pretty but doesn't recognize it until a boy tells her so
3. Characters involved in love triangles
4. The Chosen One (fantasy novels)
5. The one with horrible parents
6. The "strong' female character who only looks strong because she's surrounded by a bunch of wimpy guys
7. The brooding bad boy
Actually, if you cut these characters out totally you'd wipe out about 75% of teen/YA fiction (and a lot of adult fiction, too), but the point is to write them in an uncliched way.
One thing that turns characters into a stereotype or cliche is if their one dominant trait or characteristic is all we know about them.
As soon as you give the brooding bad boy some other qualities, for instance, he stops being a generic brooding bad boy and starting turning into an individual.
When I was writing sitcoms, the rule was that every character should have one strong characteristic, but that should be complemented by several more that might emerge more gradually.
As an example, in the classic sitcom, "Golden Girls," Blanche was the sexy one, Rose was ditzy, Sophia was feisty, and Dorothy was sensible. However, one reason the series was so successful and lasted so long was that as time went on you got glimpses of other aspects of their characters.
One good strategy, once you've established your character's basic trait, is to find a situation in which they act the opposite way--of course that has to be motivated by the circumstances. Then it's fun, for instance, to see Dorothy being the sexy one, or Rose being feisty.