From Cool Tools:
Sticker Shield is a static adhesion sheet that surrounds a decal or sticker, making it easily removable and transferable from one surface to another. I’ve always had a problem with parking stickers. External stickers are easily scraped off during Chicago winters. Internal stickers are weatherproof, but can only be removed destructively from your windshield. Unlike the previously-reviewed Un-Du and Grip Solvent, which can ostensibly remove decals easily, Sticker Shield allows you to avoid the hassle altogether and preserves the sticker for re-use.
Major (entire) thanks for this go to Nick Markos, who was kind enough to mail me a StickerShield sheet when he felt the pain of my parking sticker situation.