Think about your storage unit like a large pantry – what might you need to access most frequently? Organize your items in a way that makes sense for your lifestyle. If you think you might need to get to that box of extra small kitchen appliances more often, for example, make sure you put it toward the front of your unit.
Another tip is to "think vertically." Take advantage of the height in your storage unit, and stack things as much as possible, keeping heavier items closer to the ground. Remember, that extra side table can function as a makeshift shelf. (Be sure to cover the tabletop with a clean, soft white towel or white cotton sheet if you use it this way.) Also, make sure you leave a small path within your storage unit so you can access any items at all times. Nothing should ever be completely out of reach.
Remember, do not store any food or live plants in your unit. Perishable items can attract unwanted bugs or rodents to invade.