at least for me, and it took a combo of the right trainer and right horse to let *me* learn on and "get it". I did many if not all of the things the above posters recommend and they all help, but there's not telling what will actually do the trick for you. Last week I had a lesson with an int. judge and she said one starts with the core and seat and does not worry about the legs bouncing or hands at first. Once the core is moving correctly, one can learn to quiet the legs etc.
"Reite dein Pferd vorwärts und richte es gerade.” Gustav Steinbrecht