• Try to use boxes that are a uniform size, they’re easier to stack (remember; keep the heavy ones on the bottom and the light ones on top).
• Leave small walkways between the boxes and furniture in your storage unit so you can easily get to the items you want without having to move anything around.
• If you’re storing a lot of packing boxes in your unit, try to fill them to the top, even if it’s just with padding and old, crumpled newspapers. Boxes that are only half-filled tend to collapse if anything’s placed on them.
• If you’re putting any metal objects into storage – like lawnmowers or file cabinets – it’s best to treat them with rust protector first, or at least wipe them down with an oily rag.
• Most public storage facilities have ample security. However, it’s still wise to take a few precautions of your own against theft. Pack your storage unit so that your most valuable items are at the back, and purchase a high quality padlock to put on the door.
• The humidity in your self-storage unit can cause your furniture to warp and your appliances to mildew. Leaving a space between your stuff and the unit’s wall allows for air to circulate within the unit. Laying plastic sheeting on the floor and stacking boxes on top of wooden pallets can prevent condensation damage. So can using old linens or other fabrics, instead of plastic, to protect your stuff from dust.
• If you’re storing a refrigerator in your unit leave the door ajar. This will prevent mold from growing inside.
• Under no circumstances should you keep anything flammable or combustible in your storage building. This means no gasoline, oil, cleaning fluids or paint thinner. If you’re storing any machinery that runs on gas, drain the tank before you store it.

Source: http://www.123movers.com/guides/generalstoragetips.asp