Once the clear coat goes, and they start oxidizing, there's not much you can do for them. I took my rear one off and sent it to a guy in Pasedena and had it chromed for $400, including shipping, and he polishes the rough stuff smooth and then chromes them.
Stays clean pretty easy now.
The orange shop hand cleaner and a shop rag will do wonders cleaning the grime off of the wheels without damaging them. We stumbled on this trick after trying everything else in the shop. Hope this helps