Many of the items you store will be put away for a while. Cleaning them thoroughly before moving them into your storage unit will help maintain their value. If you need to store appliances, consider using a strong cleaning mixture with a small amount of bleach or baking soda. Leave the door slightly ajar (on a washer or dryer, for example) when you position it in your unit.
Cover the ground of your storage unit with a canvas tarp or sheets. Using pallets and/or shelving will help keep smaller items up off the ground and allow air flow around your boxes and larger items, such as furniture.
Before placing furniture in your storage unit, vacuum it with a brush to remove excess dirt. If it is wooden, apply furniture polish to seal the wood and slow the advances of mildew or mold. Cover upholstered furniture with clean (or new) 100-percent white cotton sheets. This will alleviate any transference of color onto the furniture, in the event of even a small amount of excess moisture.