Tuck your little fingers of each hand under the saddle cloth when you ride. It will give you more of an idea of just how much your hands are moving, and how much you are relying on them as you'll feel the pull of the saddle cloth each time you go to move your hands.
You can also ride with a whip laying across your hands, under your thumbs. Keep the whip even, so not sloping up/down on either side, and maintain the distance from the saddle. Having a visual aid like a whip will help keep you more aware of what your hands are doing. Trust me, I had a 'bung hand' that liked to sneak back to my hip - rode with a whip under my hands for an hour and haven't had a problem since!