I would also recommend doing exercises on those big ab balls.
http://www.megafitness.com/exercise-ball.html
like if you do dumbbell shoulder press, sit on the ball and do it instead of a bench with a back support, so you have to stabilize yourself. use light weight and start with your feet and legs apart so you get a feel for the ball. then gradually pull your feet together after each set so you use more stabilizer muscles.. also do this for db bench, seated curl, lateral raises anything you can think of that would require you to sit and use dumbbells.