I am coping to copy this information for our little horse club, because we are just starting doing small jumps, and some of the girls are having issues.
I had this same problem with my lesson horse. He has the habit of dropping his head after jumps and after a few near misses, he succeeded in unseating me. I fell hard. After that I started doing squats on the ground focusing on keeping my weight on my heels, my back straight, shoulders back, chest up (boobs out!) and chin up. I would assume this position over the jumps. It seems exaggerated but it keeps me on the saddle even over the clumsiest jumps (as well as his occasional post-jump bucks). I also do exercises to strengthen my inner thigh, which is the muscle you use when holding on for dear life with your lower leg lol.