If you try to pose, you're going to look posed. One thing I always tell a person is to be herself (or himself, depending on the person).
It's pretty easy since most of us act like ourselves for a good chunk of the day.
Take Jordan here. She's a young, angry teenager. But she's also a free-spirited hippy chick. She hates shoes, she has chipped nail polish, and she's currently not cutting her hair so she can donate it this summer.
Jordan didn't try to overdress or act like someone else during her shoot. She just acted like Jordan. And the pictures, just like Jordan, are really fun because of it.