go to a local finish masters or automotive paint shop buy a "3m eraser" and hook it up to a drill. Use this tool(3m eraser) at slow speed on a drill, to prevent damage. high speed will scuff top coat. use the eraser for vinyl and mineral spirits to remove glue and rinse, wash, wax like new!
Those are rather expensive and don't work well with old dry crusty vinyl. I'm afraid there's not much more help other than working gently to get the top layer of vinyl off with a razor blade and then using acetone to wipe off the gummy residue. But hey, what do I know about vinyl graphics? <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />
(I just finished this today:)