Everyone knows how to do crunches on an exercise ball, but those primarily work your six-pack. This move works all your abdominal muscles (rectus abdominis, obliques, transverse abdominis) plus your lower back and hips by getting circular. Do three sets of 15 to 20 reps, changing direction every five reps to strengthen your core 360 degrees around.
Setup: Sit on an exercise ball and hold a small plate at your chest. Roll forward until your back is resting along the ball and your hips are slightly lower than your head.
Move: Curl your torso up and around, moving in a clockwise circle, coming up as high as you can off the ball while keeping your hips steady and square, and lowering completely back along the curve of the ball at the bottom. Keep your chin off your chest at all times, and try not to let the ball roll around by planting your feet and squaring your hips. Switch directions every five reps and remember to breathe.
Make it harder: Hold the plate behind your head.