Build floor-to-ceiling shelves on one side of the shed. The more things you can get up off the floor, the easier it will be to find them when you need them.
Outfit the shed with a large door that opens outward. This will make much of the interior of the shed easily accessible and will enable you to move large objects in and out of the shed.
Use stacking boxes such as old milk crates to store things. Separate things into categories and put one category into each crate. Stack the crates in a corner of the shed with a sign on them identifying what's inside.