Well the best thing to think about when jumping is don't do anything. The horse should be doing all the work. It doesn't mean it isn't hard, but you want to think about being quiet and effective and staying out of your horse's way. Keep a steady and consistant rhythm and maintain your balance along with your horse's balance. When your horse jumps you want to think about keeping your heel down (no knee pinching!) and keeping it really tight and still. Fold at the hip and think about pushing your hips back and out of the way (staying centered and clsoe to the seat) and absorbing the horse's energy. As you bend at the hip let your hand follow the horse's mouth and gently rest it on the crest. On landing make sure to keep your seat out of the tack and your hand forward and don't sit up too early. It takes a ton of practice and muscle development to be able to do it and the only way you can develop the muscle is by just practicing.