Well, Flitz is probably the best product there is for nickel.
It is nearly non-abrasive, unlike most all other polishes, and won't harm the nickel unless you just get down-right carried away with it in one spot.
A slightly more agressive polish would be Simichrome.
But it is an extreamly fine abrasive and will eventually wear through nickel if you work at it hard enough.
rc
