To get on the ball, stand behind it and, using your arms, slide your body across the ball until the only contact you have with the ball is with your calves. When you do this push-up, you really have to pull those stomach muscles in. Make sure you come to a 90-degree angle with your arms. Not doing the push-up correctly is only cheating yourself. Do as many push-ups as you can do. When you start getting lazy, or your position is incorrect, stop.