With a five to fifteen pound weight (or a medicine ball) on the floor next to you, sit down with your legs extended, roll your shoulders back and sit up as tall as you can. Keeping your back straight, lean back until you feel your abs engage. For a greater challenge, lift your feet ever-so-slightly off the floor. Twist to the right and pick up your weight or medicine ball. Holding the weight, twist from right to left, reaching as far behind you as possible while maintaining your form.