Use a whip or crop and hold it with your thumbs for practice.
Relax your elbows. If the elbows are tight they won't allow your hand to follow the movement.
Grab mane and remind yourself not to let go! Doesn't cost you any money and acts as a grab strap
Get a saddle seat strap, (I know stereotypically it for newbies and beginners for balance) however a lot of people are now finding them really good practice for keeping your hands still :)
Put a crop on your hands horizontal ways