Greetings,
I posted this same question here in regard to a nickeled Colt .380. The stuff that worked for me was Flitz metal polish, whose website is listed here at TFL under "Links." Most gunshops have it, and so do online places like Brownells.com.
You only need a tiny bit, and lots of gentle rubbing, to get nickel looking like new (well, pretty darn good, anyway). After that, use it every six or twelve months.
Good luck,
Ledbetter