Put silica packs inside the safe. Silica packs prevent moisture from damaging electronics in storage and are perfect for safes. Get them free at large retail stores that sell electronics or musical instruments shipped in bulk. Go to any store that sells cameras, musical instruments or other items that could be damaged by moisture. A sales associates often will give away silica packs since they are usually thrown away. Replace the silica packs in your safe every three months or as needed.
Purchase a "moisture catcher" from a local dollar store. These items are found near electronics or home-cleaning supplies, or at some some garden-supply stores. Replace as needed, based on the humidity of the safe's location.
A box of baking soda will absorb moisture in small safes. It should be used only as a short-term solution for larger safes in humid areas. Using both a box of baking soda and a cup of uncooked rice can be effective.
Purchase a rechargeable dehumidifier for larger safes. They generally cost $20 to $40. Dehumidifiers should be charged for 10 to 12 hours, then placed inside the safe. Recharge the dehumidifier as required by the manufacturer.