Restoring Nickel plating
Okay, do not use anything like 000 steel wool or other abrasives. Depending upon the degradation of the plating, you could work through any break-through very quickly and it only gets worse from that point.
Nickel does not tarnish, but rather oxidizes and some folks tend to gain appreciation for the darkened patina that tends to develop.
If you wish to try and restore the plating to a more original appearance, you can use phosphoric acid by cutting it 10:1 parts water and acid respectively and soaking your plated materials in the solution overnight. Naturally, you'll need to take your time in removing the parts from the guitar and under no circumstances should you apply the solution to the plating while its attached.
Once you've soaked the parts, dry them thoroughly and using ultra-fine polishing compound, you should hand-rub rather than attempt to use a wheel unless you've had experience doing so. The reason is that depending upon speed, wheels and polishing compound can burn the plating, producing variability in its finish.
Also realize that if break-through is evident and the copper base has begun to react by breaking down, you'll see black pitting around areas of the nickel that have detached to some extent and rough to the touch. If this is the case, then you should consider foregoing the attempt to restore the nickel and opt to replate the pieces if you want a more clean and bright finish more representative of the finish when it was new. It should be stated here that if you are considering doing this to parts belonging to a true vintage instrument, then you risk seriously damaging its market value based upon observable characteristics which demonstrate age-appropriate curing and authenticity.
Hope that helps guide your decisions and good luck.
Mark Ostemyer, Vintage Restorations