I had a lot of trouble with my Thermarest Pro-Lite 4 moving around, and I eventually bought a quilt. Between the slipping and cold shoulders, I got tired of fighting with the pad.
If you're going to stick with a pad, I'd suggest one that has grippers. My wife's pad has little circles of a rubbery material printed on it that keep it from sliding around the tent. I assume they would do the same thing in the hammock.