Practice your two point: important in and of itself, but it also helps you really appreciate and use your joints as shock absorbers. You should be able to two point at all gaits.
Practice posting without stirrups: believe me...if you aren't doing it right, you will know when you do this. Pinching with your knees completely throws you off balance. This is one of my LEAST favorite things to do, but it really helps
Do sit ups and back exercises: this is not just for posting, you'd be amazed how much everything in riding becomes easier when you have strong core muscles.
If you cannot do the first two without balancing on the reins, have someone lunge you and practice without having to worry about your hands until you are solid on your balance.