• Pallets, Pallets, Pallets: Keeping your items off the floor is almost essential. It might sound silly, but before renting your unit, there’s no way to promise that the melting snow won’t come under the door, or the adjacent unit won’t have a spill that soaks its way into your heirloom sofa.
• Label, Label, Label: Even though you know exactly what you’re putting in your storage unit at the time you open it, that doesn’t mean in 6 months when you need back in it that you won’t be digging for ages and opening random boxes until you find what you’re after. Label everything. The hidden, the obvious, and you can even go as far as to make a checklist that hangs inside the door telling you where something might be (back right corner under the desk = cameras to sell on eBay).
• 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 storing a refrigerator in your unit leave the door ajar. This will prevent mold from growing inside.

(Tips a compilation from http://www.123movers.com/guides/generalstoragetips.asp; http://www.apartmenttherapy.com/chicago/tips/9-tips-for-using-rental-storage-units-124239)