Plan your storage space. Place those items that you need to get to often in the front. Allow for walkways. 

Completely fill each box with your heaviest items on the bottom and lighter items on top. 

Try to keep your boxes in uniform sizes. They are much easier to stack that way. 

Label ALL boxes for easy identification. 

Leave a small space between the wall and your storage items to allow for air circulation. 

Use a HIGH QUALITY lock, preferably a "disc lock", for your unit. They are much harder to break or cut keeping your valuables as safe as possible. 

Make up an inventory of your stored items and keep it in a safe place at home or in a safe deposit box. 

Use uniform size boxes for easy stacking. BE SURE they're strong enough to hold 25-30 lbs. (It is best not to put too much weight in each box.) 

List contents of boxes on all four sides, number the boxes, seal with masking tape or glued paper tape. 

Stack lighter boxes on top of heavier boxes. 

Dishes and glasses should be wrapped in packing paper and placed in dish pack boxes. Stack those on top. 

Place your storage items on 2" x 4" boards, bricks, or pallets for maximum air circulation.

Drain all gas and oil from engines in lawnmowers, tillers, etc.

CAUTION: It is against the law to store materials that are HAZARDOUS or COMBUSTIBLE such as gasoline, solvents, paint, etc. Drain all remaining gasoline from lawn mowers, power tools, boats, snowmobiles, etc.

Pictures and mirrors should be wrapped in mirror crates marked "FRAGILE" and stacked on end.

Furniture with drawers can be utilized as storage places for knickknacks, china, silverware and small items. Wrap in tablecloths, towels or blankets to prevent breakage.

Larger appliances make excellent "packing cases" for blankets, towels, tablecloths and clothes.

Shovels, hoes, rakes and hoses can be stored together in empty trash cans. Stack extra cans inside one another.

Tables with removable legs should be "broken down" to save space.

Sofas and loveseats can be stored on end to save floor space. Always place a protective waterproof cover under the end on the floor.

Cushions should be wrapped in plastic and placed on top.

Cover stuffed furniture with dust cover to protect against dust.

If you stand mattresses on end, be sure to "prop" them up to stand straight or they may bend out of shape and become lumpy.