A wipe with WD40 gets the worst of the crud off and then a polish with Autosol
If you keep your ends clean you don't have to do it that often.
I have found that the GTI ends get very dirty, very quickly. Most of my other cars have been fairly easy to keep the tail pipes clean, but my GTI seems to be almost impossible to keep them silver. Two days after polishing them up, they're black again