Lee Valley Vice Rack Stop
This simple device from Lee Valley is designed to help prevent racking when clamping work at one side of a vice. Simply stack up as many of the 0.10" thick shims as you need to match the approximate thickness of your work, up to a total of 4", and insert it into the vice opposite the work being clamped.
For thicker work, you can add a piece of scrap wood to occupy the bulk of the space, and then...