Any advice on cleaning this? I just got it, and I've never had a nickel finish before.

Mother's Mag & Aluminum polish works great on nickel. You can get it at any auto parts store.
Works WAY faster than any other polish I've tried.

Never Dull Works wonders also. Its much less abrasive. I have never used it on Nickle but I used to have that same frame when I was growing up. I had tried many things on it. The great thing about never dull is if it doesn't take the print off of stickers. You can keep stickers on and clean bare metal up great. I have used it on original stickers before with no problems but be careful of course. Those diamond stickers might not fair as well.

Nbl 49a
Kastan National Team 91-92