Well, the cowboys I know would laugh at the mere thought of posting. I am not talking cowboy wannabe's but true cowboys who use their horse year round. And you can not post at a show or you will be disqualified.
I have only ridden western so don't know any other way. But the easiest way to help you is to make sure you are sitting so that you are going to allow yourself to move back and forth instead of up and down. You need to relax your muscles to sit a trot and allow yourself to move with the horse :) If you start to bounce stop your horse and start over. If you start to bounce you will stiffen up and the bounce will only get worse.