Hanging from a pull up bar with absolutely straight elbows is one of the keys to shoulder flexibility AND a good kip. Practice just hanging from a bar and when you do knees-to-elbows, don't bend your elbows.
1-2-3-4-4-3-2-1 weighted pull ups
between each set do 10 push ups
- choose a weight for pull ups that makes it difficult to do 1-2 reps