Line the floor of the storage area with plastic sheeting. This helps prevent moisture from seeping in.
Lay wooden pallets in the area where you want to store the sofa. The pallets will raise the sofa off of the floor, allowing air to circulate underneath, decreasing the likeliness of mold.
Place the sofa on the wooden pallet. You could store the sofa on its side to maximize storage space, but this can wear out the joints in the sofa. If you have a heavy sofa or will keep the sofa in storage for a long time, it's best to store it in its upright position.
Cover the sofa using a drop cloth. This allows air to circulate while preventing dust from settling on the sofa.
Place other items around the sofa. Keep in mind that certain items might bleed color onto the sofa, so keep these items away from the sofa.