From my attempts, the best thing to do use is rubbing alcohol. But if you have really heavy residue like stickers or old duct tape then you will probably find it easier to start with goo gone get most of the gunk off then clean it again with alcohol.
But, scratches and scuff marks there is basically nothing you can do. If you have an extremely bad gouge, you might sand it with sand paper, but this would only be to make it smoother if it the scratch has a sharp (dangerous) edge, it would not make it look any better, it will probably look worse.