After investigating how to do it on the net, since I didn't have access to all of those fancy polishing pastes nor solutions which dissolve the coating, I polished my Ray-Bans with Colgate Triple Action toothpaste. Result? Flawless!
Toothpaste has been used for ages to correct minor scratches on CDs and as we all know, CD surface is very fragile. Try it for yourself on a CD and see if your toothpaste scratches it and if not, I would say it is safe to use on your sunglasses.