First method is the easiest. When you are swinging forward, point your legs straight. On the way back, curl your heels towards your butt. By doing this, you actually work your legs and abs a little bit. Alternatively, you sit still on a swing, hold the chains, and then lean back. Extend your legs straight with toes pointed forward. Cross our left calf over your right one and contract your abdominal muscles to keep the swing as still as possible. Keep doing this until your abs are exhausted.