A lot of those stickers can be taken off by spraying them with windshield washer fluid. Keep it wet; you can lay a paper towel or clean cloth over it. Let it sit for a while, it will dissolve/loosen the glue. Wet a thin scraper or putty knife, then slide it under the sticker. You'll have some time to do something with it, maybe relocate it to a different window/location so it doesn't block your vision. Put it where you want it, and let it dry in place. I've also seen guys put old stickers on display boards to keep with the cars, then put the board in the window when they go to car shows.
