Consider awkwardly-sized items that won't stack well, how often you will need to use them and how heavy each box is.
Calmachie Storage can be quite helpful in determining how much space you will need.
Use boxes instead of plastic bags, as they are sturdier and stack well. Always invest in and use high quality packing materials.
As you arrive at the facility and begin to unload, arranging your belongings in your storage unit will make a noticeable difference in maximizing its convenience to you.