Leaning back will throw you off balance, especially when you land. Try to stay completely vertical. Of course this is more difficult, and your kick won't be as high initially, but keep practicing and stretching. Also, if you're kicking a target, Try to jump forward at the target rather than walking up close to it before executing the kick. You'll find you actually get a higher jump when jumping forward, as well as forcing you to remain upright. Plus, in sparring and in real life, your target is rarely close enough for you to jump straight up and kick. You usually have to close the gap if you want to make contact, and your forward momentum is a great way to get more power without exerting more energy. (P.S. I also love practicing the jump and spin without the kick. I never would have developed good jump-spin techniques without practicing this way. Also try just the spin kick on the ground a few times before incorporating the jump. If you break down the technique (any technique) into its basic parts, I find that it's much easier to execute the whole thing later.) Happy Training!!