Swiss Ball Pikes
Assume a pushup position with your arms completely straight (your hands should be slightly wider than, and in line with, your shoulders). Rest your shins on a Swiss ball so that your body forms a straight line from your head to your ankles. Without bending your knees, roll the Swiss ball toward your body by raising your hips as high as you can. Pause, then return the ball to the starting position by lowering your hips and rolling the ball backward. Do three to four sets of 8 to 12 reps.
Source: Men's Health