I keep a dehumidifier in my gun storage room. Whirlpool. And a hygrometer to help monitor humidity levels. I oil the internals of the guns that aren't used often. Once every year or two. Oil the internals of guns I use after each use.

I put Renaissance Wax on the externals of guns I care about...usually old blued guns. Oil on externals of guns I care less about - polymer guns or fairly worn finishes. I do keep my handguns in gun rugs mostly, but with the other precautions I haven't had rust issues to speak of yet.

Probably the best thing I've done so far is get my basement waterproofed. It used to flood after heavy storms during wet spells. YIKES!

Sometimes I wonder if those big heavy duty safes trap in moisture, too. I have ventilated steel security cabinets, locked and bolted to the wall. Having a fan run in your gun room might help, too.