Samantha Clayton, a former Olympic sprinter and personal trainer in Malibu, Calif. loves this move for flat abs: "The stress you put on all of your core muscles in order to stay in this position forces your abs to be contracted the entire time."
To do it:Get into pushup position with both hands on the stability ball directly below your shoulders (your feet can be slightly wider than shoulder-width apart to give you extra stability). Contract your abdominals and try to bring your body into a straight line from your neck all the way to you toes (your butt should not be sticking out, so contract your glutes and keep your hips down). Once you are stable, lift your right leg a few inches off the ground and hold for 10 seconds. Repeat on the other side. Try to do three sets of a 10-second hold, and then advance to a 30-second hold.