Boxes are sturdy. They stack well and they are generally better than plastic bags, as plastic bags can trap humidity and cause mildew damage. Be sure to fill the boxes to the maximum capacity, as half empty boxes can collapse or damage your belongings while in transport. Distribute weight evenly in boxes and make sure nothing is too heavy. Also be sure to stack heavier items at the bottom and place fragile items on top to prevent damage.
Consider which items you might want to access frequently, and store these items in the front of the unit. Use specialty boxes for items such as artwork, mirrors, electronics, lamps, dishes, glasses, mattresses, sporting goods, and musical equipment.
Use plastic bins to store items like clothing and blankets, as these containers are much more efficient at keeping out pests and smells from nearby units.
Storing canned food is generally okay, but don't store perishable food items, as these are known to attract pests.