If you, or someone you know, take certain medicines, they have the small tubes or bags of Desiccant in the bottle. I save these, when I can get them, and throw a few in the gun case with a gun. I think they're putting them in aspirin bottles now too.
However, the larger bags thats sold last a long time, and you just throw them in drier about once a year, then back into the gun case. They will catch any moisture from temp. changes and humidity that get within the case. I use these in my gun cabinet also.
Last, a good coat of wax on the metal surfaces will help a lot. Just let it dry, and use a cloth to buff it off, just like applying it to a car.