Ballistol works best.
Don't use anything with Ammonia - not even Windex.
At the very least, the ammonia will turn the finish yellow.
Most gun solvents, like Hoppe's #9 have ammonia.
Ballistol does not & it's 100% safe to use.
Don't use Flitz or any polish that might be abrasive - including toothpaste.
I'd avoid Brasso also since it's nickel, not brass.
Johnson Paste wax can be applied as can Ren-wax to help the finish repel fingerprints.
I have, at last count, about 8 nickel plated guns.
There's no real trick to cleaning them - as long as you stick with Ballistol.
Heck, you'll probably find out like I did that Ballistol works so well, you'll use it on everything.