Lay on your back on the floor with your heals elevated and resting on the middle of the exercise ball. Push your left heel into the ball, drawing it toward your hips. Roll the ball under your foot so that you can dig your left toe into the ball. Meanwhile, elevate your hips and kick your right leg back to the floor behind your head. Switch legs then repeat. "Once you have mastered the movement, you need to constantly increase the speed. It's more important to maintain the quality of your form at a high level than to just do multiple repetitions," MacMillan says. Once the form starts to get sloppy, move on.