Take a look at Brittany's Muscle Up. She's been working on this movement since she started the gym. It is definitely not an overnight thing for most people. Notice a couple of very small things that are keeping her from completing the movement.
1. Her initiation of momentum is good.
2. Upon full kip, she is compensating with her left side. This is causing an imbalance and excess use of her left side. This is also going to cause eventual tendonitis with prolonged (months) of use.
3. At the top, she needs to keep her hands together as much as possible so she can get on top of the rings.
Use your legs to kip, but only enough that it adds to your hip movement. Too much leg make you platform under the rings, and that means more work, not less. Awesome job Brit! You'll be there any day now!
20 Push Up/Sit up
10 Push Up/Sit up