Check the March 2007 issue of Bicycling Magazine. It lists 8 sweet exercises for your core. I’ll show you some pics of me doing these exercises, just don’t laugh at me being out of shape!
There’s also some websites with more info:
http://exercise.about.com/cs/abs/l/bl_core.htm really good core exercises
http://coachlevi.com/bodyweight-exercise-collection/ this guy shows some core exercises