I still have the stock exhaust tips, but what I do, is after every drive yep, every drive I take a damp microfiber towel and wipe the insides of the tips, and use a dry microfiber towel right after to each tip to buff em clean - They look perfect
Getting them looking perfect in the first place is a different story... 0000 steel wool, P21S Polishing soap, and elbow grease