butterfly situps, 4-count flutter kicks, leg levers, knees to elbows and toes to bar.
the last two are difficult and probably produce the best results. hang from a pull up bar and bring your knees up to your elbows or your toes up to the bar. once you get strong enough to control your whole body’s movement you can bang em out in a kipping motion. before you get that it’s difficult and slow and you’ll have to take lots of breaks to bang out 50 or so.