As a pretty new male rider, I'd say first thing is fairly tight, supportive underwear. Jockeys, not boxers. And by "tight" I don't mean smaller than the size you normally wear, but new enough so that they are still elastic. Or a jock strap.
Second thing is starting with a standing trot, so that when you bounce, you don't go all the way down and hit the saddle. (I think this is easier on the horse, too, as your legs act as shock absorbers.) Eventually you will pick up the rhythm of the horse's motion, and become able to follow it so that you don't bounce.