I agree with what littlelady said and am adding on with these thoughts. Most of the kip problems that I see are due to a glide that is cut short. Make sure that you have a nice strong glide swing and that you stretch it out until you feel the bar tugging on you. Also, when you do your V-ups, start with your arms over your head. When you sit up to the V, snap your hands down to the floor and push down. The goal is to be able to hold you bottom off the floor while holding your legs up. So many girls get the kip, but can't hold their feet up at the end. If your feet fall back too early, it puts extra weight on your arms and forces you to fight for the end of the kip. It also makes having a nice cast out of the kip difficult. When you have an open bar, work on drop kips -- start in a front support, lean back and pull your knees to the bar, the kip back up. These take lots of strength and good technique, so they will help build your muscles as well as work the kip technique.