:...best way to remove an old bumper sticker..."
Best way - don't put it on...
I tape them to the inside of my rear window.
I NEVER put bumper stickers on my bumpers. And If I have to have a window or windshield decal, I carefully put clear plastic on all but the outer 1/8", then tape it onto the inside with 1/4" width tape.
My cars always look like they just came off the showroom floor as long as I own them. Even my previous beloved car (lost to a rear-end collission) after 120K miles looked like it was brand new.