I always squeeze my saddle with my thighs, keeping my lower legs loose and free to aid, and making sure not to squeeze her belly asking for more speed. "Pinching," as we call it, can help your butt keep contact with the saddle. Try that for a while, and with time you'll be able to squeeze a little less and a little less, until you just sit it naturally. My mare has a very very smooth trot, but one of the 4 year olds I'm finishing has a trot like an army tank. It can be quite painful if I don't pinch.
Best of luck!