Get the most bang for your buck when it comes to your storage unit—make use of every inch of space. Stack boxes from floor to ceiling. This way, you are taking advantage of the vertical space available in your storage unit. Do use caution when stacking boxes, placing larger, heavier boxes on the floor, then stacking smaller, lighter boxes higher.
Are you storing furniture, such as a dresser, a chest of drawers, or a bookcase? Fill those drawers and stock those shelves. Just because these items are large, doesn't mean you're necessarily losing out on space for smaller items. Use the floor-to-ceiling principle here as well. Take advantage of the flat surfaces of your furniture to hold more boxes and other items, allowing you to utilize your storage space to the fullest.