I think about putting my hands near the martingale and push my heels down. But don’t think too much on your position, if you focus too much then you won’t jump as well. A lot of two point practice while flatting makes a huge difference as well, since it gives you the chance to focus on it while your horse warms up.
Don’t worry about you position over jumps !! I did it for ages and just concentrated on my position and not going clear or setting the horse up correctly. The main thing is to think about is making it over the jump!!! Don’t stress about little things like that, because they don’t make a massive difference. If you are still concerned and it stops your horse from going clear then ask a riding instructor xx