Join Date: Sep 2007
Location: Washington State
I don't know if you can afford to do this with your horse (if his temperament will allow it) but one exercise my instructor had me do once to stop me relying on my hands while posting was to let the reins out pretty slack at the trot and post that way. I didn't even realize how much I was using my hands to balance until I had to do that.
That's not all, though. She made me work on posting to every other stride... so I had to stand in the stirrups for two strides, sit, stand for two strides, sit. All without touching my reins. You can also try to "glue" your arms to your sides to keep your hands from moving all over the place.
Also, I can't tell for sure, but it looks like your hands might be turned in more of a "piano hands" position rather than with your thumbs up in the more desirable manner.