As dumb and fundamental as it sounds, the best way to reduce and keep humidity down is to use a dessicant, a fancy word for something that takes moisture out of the air.
I burn coal, anthracite, for heat during winter and in the off season, and have the unenviable task of removing the sulfurous/acidic ash that corrodes the heat boxes of both of my stoves. As a chemist, I found that removing moisture/humidity worked wonders in avoiding corrosion (exfoliation of the boiler plate steel) out of the fire box of both of my stoves.
The simplest way of doing such is to buy magnesium sulfatehydrate (Epsom salts) from Wal-mart, placing it (~5 lbs) in an aluminum pan and heating it at 450F in an oven for 1-2 hrs. This drives the water out of reagent and the anhydrous magnesium sufate acts as a sponge to remove water out of the firebox during off season. I applied this concept to my gunsafe and it worked gangbusters. I start this in May and re-heat/recharge the mag sulfate in August. It's better than the "Goldenrod" to suppress corrosion and pitting
