For your lower leg try tying your stirrups to your girth using bailers twine. I had to do it for a while and it really helped. For your body try going up to some jumps in your 2point. Or even doing your 2 point without stirrups while flatting
To keep a good position during the jump of your horse and on the back side of fences, you have to keep your pelvis slightly forward to have a better balance and stability of the leg.
You have to keep your calf close to your horse (your knee open) and put as much weight as you can on your heels.
During the descending phase, your back has to be straighten up not to disturb the balance of your horse.
To help you do that, you can jump without your stirrups to keep your legs at the right place . It will also help you to wait for your jump and not throw yourself on the fence (it is often linked and make legs move)