Rubbing alcohol should also do the trick.

Whenever I get those stickers, or airport security stickers, I immediately loosen them at one of the corners so they're easier to peel off later. And I remove them as soon as I pass security or make it onto the plane. They never leave a mark if you get them off before the glue has a chance to dry out.

However, when I received my most recent passport, it had a giant sticker affixed to the back with a QR code to scan for "helpful international travel tips" - no thanks. That sticker had probably been on the passport book for months, and I couldn't peel it all the way off. Tried a few applications of dish soap and rubbing with my fingers with hot water and was able to get most - but not all - of the sticky crap off. The alcohol took care of the rest.