Blue Magic is of course what PRS recommends. I’ve had pretty good results with Flitz metal polish.
For prevention of tarnish, I’ve found that fingerprints tend to accelerate it. When I notice fingerprints on the pickup after I play, a microfiber towel that’s a tiny, tiny bit damp - by that I mean not damp enough to drip, almost dry - followed by a dry microfiber towel immediately, keeps the pickup covers clean. Good microfiber toweling won’t scratch the surface, though the 58/15s come slightly ‘scratched’ from the factory (on purpose).
I keep mine cased, and they’re always shiny, but I don’t know if the case has anything to do with slowing down oxidation.
I used to be fussier about that stuff than I am now, for some reason.