My tip is to work a lot on combinations, it will help you to have better reflexes and sit down quicker after the jump.
You can do this with or without stirrups (to work on your back position and leg one).
Be careful not to sit down too quick before your horse ended the jump.
To be really comfortable while sat, the best way is to work without stirrups (i know it's hard!) but you can also try to ride with a dressage saddle to feel the right position.
I hope i could help you :)