Try not to get the horn or antler too hot when your grinding it on the belt sander. They'll sometimes warp. Also, if you're dealing with a small piece while belt sanding, it can be difficult to hang onto. I've spent hours looking for the little overlay that shot out of my hand. I've found using double stick tape work quite well to hold them onto a small block of wood while you're grinding them. Just don't pull too much pressure on the belt.
Once the glue surfaces are flat and ready to glue on, I use the wilder rubber bands to hold them in place. Using clamps on the top angle of the overlays is an exercise in frustration. With Super Glue, you can probably just hold them with your fingers while it sets up. I mostly use TB III