Well, it just takes some time and experience to get the feel right, so don't worry if you bounce around some. Everyone does in the beginnning.
YOu think about your butt being kind of glued to the saddle, so when the saddle goes forward, your butt must also go foward. Your job is to ride your own butt, since it is locked with the saddle. So, the more you grip with your legs, the harder it is for you to stay balanced over your own butt. Hard to explain, but basically, sit up straight, try to be kind of "heavy" downward into the saddle, and don't grip the horse with your lower legs.