Remember that old workbench with the end vise? I showed you how I keep the jaws parallel by using spacers equal to the width of the thickness of the work you're trying to hold. I also use this trick. I make an "S" shaped piece as in the picture. One side is 3/4 " thick and one side is 1/2" thick. This works easily since most of the time I'm trying to clamp wood of this thickness.