Try standing a little in your stirrups, with your upper body inclined a little forward. This is an english two point position, and can be done in a western saddle too, just be careful of the horn. If you are riding western, you can put your hand on the horn to help you balance at first. Find the place where your feet are under you and you can stand comfortably, and use your legs like rubber shock absorbers. This is a very stable position, and lets you get off your horse's back so you won't feel like you are bouncing off. Also check out the post on riding the sitting trot on
www.horsetrainingquestions.com.