Whenever I use pipe clamps, I like to place a block of wood between the clamp and my workpiece. This clamping pad helps prevent damaging the surface of the wood. The problem is trying to hold the block in place as you tighten the clamp.
To solve this, I made some magnetic clamp pads. Each pad is just a block of wood with a shallow hole drilled in the center. Then I epoxied a small magnet into the hole, as you can see in the drawing. The magnet holds the pad in place, leaving me with both hands free to align and tighten the clamp on the workpiece.