I just went through this and finally got my eq over jumps to not look like a sack of potatoes. One thing I did was get someone to video my jumps and I would look back at them after I took them and try to fix what I did wrong. I don't know why this worked so well for me, but it's worth a try.
practice courses and eq over ground poles. fun and less stressful practice for you and your horse
Practice holding yourself still over your horse in your position at the walk, trot, then canter. Small bounce jump exercises really help you feel where your position is as well
I think it's really important to be in physical condition yourself. Some good thinks for riding are squats, and a versa climber is awesome!
Evention TV has a really great video on junping form !
My trainer suggested to me to stand on a stair or something you can put your heels down on and to practice pushing my seat back to ring a "bell", when keeping your back straight and moving your arms forward with each rep. Doing this will help your balance and will make this form your reflex. hope this helps!