Hi Gypsy - hope you're having fun!
Couple of quick exercises that really helped me.
> Practice moving from your usual riding position to the jumping position at walk... so something like 8 paces seated, 8 paces jumping... etc, then speed up! I find this particularly effective at the canter. If you find yourself getting in front or behind the horse or leaning on the neck, then slow down a pace.
> If you have someone who can lunge, then practice some small fences with no reins. I found this helped no end with my posture.
Also, it might help if you imagine your weight flowing down the front of your legs to the balls of your feet - helps to keep your weight centred. Then push heels down to anchor the position.
Do you have a friend who can watch and give a few tips? I always think it's best when jumping anyway.
Hope you have lots of fun!