This is going to sound a bit weird, but once you've used a bit of white spirit to loosen them up (and scarpe them off with something plastic, as they can be very stubborn), a cheap pack of face/hand wipes from Tesco will get most of it off. When I took my numberplates and bumpstrips off, and found 13 years of glue and gunk behind them, it cleaned them right up (Y)