There is a kit in a spray bottle that Autozone carries for about $7 bucks. Real simple, spray the stuff on, let it sit, then peel off (comes with a razor blade/tool). Should be enough to do 3 or 4 cars. Takes an hour or two at most. Then clean the windows with windex or soap/water to remove any leftover residue.
Edit: Here’s the link: http://www.autozone.com/servlet/UiBroker/?UseCase=C003&UserAction=answerQuestion&Parameters=Film+Adhesive+Remover
