Lie on your back with your hands behind your head, knees bent and your calves on a bench. Have a partner hold your ankles. Do 10 full sit-ups, bringing your chest as close to your thighs as you can. Do 10 more, bringing your right shoulder toward your left knee, then do 10 on the other side. Repeat the sequence, doing five reps in each position. Finish with 10 reps straight up. As you improve, increase your speed. You can also hold a five- to 15-pound weight plate on your chest.