Take a class ASAP. Your body position is very poor. That being said, when riding agressively (track) your butt basically only touches the seat during straight aways. You need to lead with your chin/head/shoulder a lot more. Work on getting your forearm inline with the grip and your head down by the mirror. For now you don't have to work on moving your butt, but make sure your getting your upper body way over. In the last picture it actually looks like your leaning the wrong way.