Where To Begin
The best way to maximize your storage space is to start at the bottom and build up. Heaviest items on the floor. Many people like to lay carboard down on the floor to reduce natural moisture. Leaving an aisle for easy access to all of your items is also key.
Think About Shelving
You may want to purchase large plastic or metal 'stand alone' shelving units to stand on the perimeter of your unit to help organize and provide greater visibility of your unit.
Use Furniture To Maximize Your Space
You can stow furniture more compactly by taking it apart as much as possible. You may also save space by packing small items in drawers or putting boxes underneath tables.