Getting them hot with a hair dryer usually works and then use bug and tar remover to get the gunk off that is left behind!

 

for the ones on the glass, I used a razor blade and carefully lifted up a flap. The car was new enough that once I had a flap to grab onto, the sticker came right off and did not leave any residue.

If you have any on the paint, it is a good idea to heat them up first with a hair drier, then do the same as above. Follow after with goo gone or a similar product.