Sit on the edge of a bench. Lean back until your torso is at a 45-degree angle to the bench. With your knees bent (or straight if you can manage), lift your legs so that your torso and thighs form a V. Tighten your abs, keep your upper body still and slowly move your legs side to side like windshield wipers. Do two sets of 30 reps (left to right is one rep).