Just make sure you are always aware of where your hands are. And your trainer can also tell you whenver your moving them too much which will help you as well!
Hey! Holding a crop helps a lot, all you have to do is hold the crop horizontally with both hands. If you are at a show, grabbing a little bit of main also helps a lot while riding
One exercise you can practice is holding a crop horizontal while at the same time holding your reins. Put your left and right hand on either end of the crop, make sure you hold the crop the same way as the reins. It’s awkward to hold your reins and the crop so it will make you more aware of where your hands are. You’ll feel if your hands are moving too much, if they are falling to one side etc. There isn’t one exact exercise that will help you with keeping your hands still, it will just come with time and practice. Hope this helped!