Usually you can bring them back with some wire wool and something to clean them. Not much of a detailer myself but i expect someone will soon be along to let you know which product works best.
I think people use Autosol metal polish to polish their tips :roll:
A guy called MachineMonkey on here makes some nice stainless steel exhaust tips for a good price. They seem to be popular and look great from what I have seen.
Yes have been using Autosol. Its good stuff but the tips have some light cracking on the chrome or finished areas so autosol won't 'polish' this damage out.