Just bend at the hips and keep shoulders back. Heels down too. I just think about each thing you need to be perfect and kinda make a little checklist in your head that u have all of them. Done.
For smaller jumps (below a meter) make sure you're just bending at the hips, not getting out of the saddle. I used to do that a lot, and I looked so wierd because I would like jump outta the saddle for these small jumps. Lol
My trainer made me do all of my jumping lessons in a bare back pad. It has really helped with strengthening my muscles and improving my balance. My position has really improved so I do highly suggest it!
Stirrupless work can work. If you're up to that.
think of you moving little and let the horse come to you, think about putting your belly botton to the pummle of the saddle (only a little) and let the horse do the rest!
To have a good jumping position, you need to first lower your heels. Don't compress your knees on the saddle! Your hips have to be right above your feet.
Keep your look ahead and don't fall forward.
The first times you can hold on to the mane to keep a better balance.
My advice would be to train a lot on small jumps and will see that your position will progress every time!
Good luck :)