A little heat from heat gun or hair dryer, fingernail polish remover (same thing as acetone), & WD-40 is what I have good results with. Saturate the stickers with what ever you use. You have the get the solvent between the sticker and the paint to loosen the adhesive. I usually don't use a putty knife of scrapper, but if you do, use a plastic one rather than metal. Metal will scratch big time. I usually saturate the stickers and wipe with paper towels. Might take a while, but no scratches or paint damage. Of course after removing stickers with solvents, rinse the painted surface very, very well. I've also used gasoline, kerosene and bug & tar remover. As long as you have a hard paint finish it should be O.K. The main trick is to saturate and get to the solvent to penetrate the adhesive behind the sticker.
Good luck. Hope this helps !