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.

 

 

The jaws are kept parallel, no problem!