Place a pallet on the concrete floor.
Stack boxes or furniture on top of the pallet.
Drape a plastic tarp loosely over the stored items to prevent water damage due to condensation dripping from above.
Place an oscillating fan in the room. Air circulation helps prevent sweating, mold and mildew.
Place a drying agent, such as a calcium sulfate product, near the stored items. Calcium sulfate absorbs water from the air and therefore reduces the humidity in the room.
Place a dehumidifier in the room. Empty the water pan regularly.