There are certain solvents that will affect a nickel finish (usually with ammonia in them). Just look for one that says "safe for nickel finishes" and you'll be fine.
They show fine scratches more than a blued gun, so go light on abrasives and scrubbing. I use Renaissance Pre-Lim and Wax to make nickel guns look nice and stay that way but there are plenty of other good options.
Hoppes and similar copper solvents that contain ammonia will damage the plating.
Use something like CLP or dozens of other better cleaners. My personal choice is Kroil.
Polish once lightly and sparingly with Flitz or Mother's and a micro fiber cloth.
As stated, Renissance Wax is a great finish: