I've tried a few things that have failed. I think because it's acrylic. Mine has stress fractures which dont help with the over look. I can usually get them looking nice for a couple of days, then they look faded and oily as products don't soak into that type of plastic like they do normal "trim" plastics.
Actually writing this has made me wonder if something like plastX would work, as it does a good job on headlights, then maybe some UV headlight protection ontop. Might try it sometime. Failing that maybe a polish and a coat of UV resistant Clear.