1. clean
2. magic eraser the crap out of them. make sure the eraser stays lubed with the cleaner or its like fingernails on a chalk board.
3. rubbing alcohol. this will put a little bit of the shine back.
4. clean again
5. novus plastic polish. the #2 fine scratch remover
6. clean again
7. novus cleaner/protector
8. rock out! your done!
if you want some of the deeper scratches to come out then you can work it just like paint on a car. start with some 800 grit then to 1000 then to 1200 and so on to the polishing steps. unless your building a show quad its not worth going to those lengths to make plastics perfect when your just gona tear them back up anyway. imo