Another important thing is to make sure that your stirrups are the correct length. Riding with stirrups too long can make you lost your base of support causing you to throw your body forward to catch up with your horse in the air. Jumping ahead is definitely my biggest jumping fault I have to come back and fix time and time again. Shortening my stirrups as I jumped made a big difference, working on what MIE said (particularly with gymnastics) made a bigger difference. Good luck! It's a terrible habit to correct isn't it?