goo-gone, wd-40, citrus based cleaner, green scrubby, steel wool, grinder, belt sander with 90 grit paper......haha
seriously, goo-gone is your best bet, once you remove the sticker soak the residue with the goo-gone and letit sit for a few minutes then take a rag or towel and wipe it off, might take a little elbow grease, but it will come off
I believe that aircraft adhesive remover works the best. They also make a sticker eraser that goes on a drill or grinder that is made out of rubber. It doesn't hurt the paint and takes the sticker right off.
I used to work at a trucking fleet and when we traded in truck we had to take all the stickers off and thats what I used. Snap-on makes a real good sticker eraser tool.