Heat from a blow dryer and those plastic putty knives can work well. Or kettle of hot water. Heat softens the adhesive. Plastic putty knife won't gouge surface, might want two or three the edges don't last all that long. But they are cheap.
These 3M stripe off wheels are erasers that go on a power drill. They are incredibly fast and easy. I did every pin stripe and decal on my scamp
in less than 2 hrs. Little pricey for two bumper stickers but for ease of use they can't be beat. Just make sure things are cleaned, don't want to rub dirt and leave scratches.
3M Stripe Off Wheel 07498, 3/8" Mandrel, 4" Diameter x 5/8" Width, Beige (Pack of 1): Power Rotary Tool Accessories: Amazon.com: Industrial & Scientific
Also available at auto parts stores, especially auto paint
I have used goo gone (at least that is what I recall it being called) on mine for caulk with no damage, don't know how it would do on stickers and I was applying near the belly band not out in the middle of the surface.