On glass, heat would probably work well as mentioned, but I've worked at a car dealership for years, and always removed stickers and window paint with a simple straight razor blade and windex. The windex is really just for lubrication - spray a bunch around the sticker, and scrape it off using the razor blade at an angle. Any remaining residue can be cleaned up with more windex and fine steel wool (the finest you can find). Detailer's trick ;)
Word of caution however, DO NOT DO THIS if it is on the inside of the rear window and you have defrost lines, OR if you have aftermarket tinted windows, as razor blades and steel wool can easily ruin both.
If either of these are the case, you are in for a bit more of a hassle, and I would suggest just sticking with just heat and windex or some sort of goof-off and a rag to work at it.