On any gymnastics apparatus, head position is everything. If you throw your head back trying to do a back flip, your face will meet the floor very quickly! Where your head goes, the rest of your body will follow. You can use this principal to your advantage on kipping pull ups by driving your head as far forward as possible on the down swing, and as far back as possible on the upswing. The path of your head in relation to the bar should be a large letter C. At the very bottom of your swing drive your head forward between your arms and then use your momentum upwards and backwards to pull up on the bar. At the top of the bar, push your body backwards away from the bar instead of dropping straight down so that you have momentum to extend your head forward between your arms on the downswing. The bigger the swing and kipping motion the less work you have to put in!
To help with your head drive forward at the bottom of the bar, continue working on shoulder mobility exercises with your arms held shoulder width apart.