There's not much you can do about that. If you use a quite abrasive polish you'll probably get some shine back, but you're also rubbing away the plating - and in any case the problem will come back quick enough.
I don't know whether nickel-plated hardware normally has some kind of protective layer of lacquer/polish, but if it does PRS don't use it - their nickel hardware seems to tarnish exceptionally quickly! The bridge and pickup covers on my McCarty are already pretty tarnished, even though the guitar's not been played much. And I have very non-Rory-Gallagher-like body chemistry, the hardware on most of my guitars looks new and I never get corroded strings.
I think it's one of those things that's annoying, but you just have to resign yourself to it. Once it gets more tarnished it'll look pretty cool.
