Theres a product called Goo Gone out there, that and a razor blade man.....
if your takin it off the back window just be careful for those little lines
a heat gun would also work well but how many people have one
If your taking it off the sides use goo gone, mineral spirits, etc and a thin razor blade. off of the rear window.. LOL it's fun I used ammonia with plastic and I sprayed ammonia on the rear window the put the plastic up against it so that it is held on by the ammonia. leave this sit for about an hour and make sure your doing it in a well ventilated area. then come back with a rag and wipe it off. The rear window is a Pain! either way!!!
Simple green. Let it sit on the glue for a few minutes, then scrape with a NEW razor blade. Any excess residue can be removed with a non-scratch scouring pad.