When using clamps, it's a good idea to sandwich wood spacers between the jaws and project to keep from marring it. But how do you keep the scrap in position while tightening the clamp?
woodworking technique shop tip

Recess small, round hobby magnets into 2" squares of 12 " plywood. Drill the recess holes just deep enough so the magnet projects slightly above the face of the block, then epoxy the magnet in place. The magnets hold the blocks on the clamp jaws while you're clamping.
—William M. Resnik, Albuquerque, N.M.