Before installing storage units, lay down cinder blocks, concrete piers, a concrete slab, or at least crushed stone. But don't stop there. Make sure the supports underlying the floor are made of pressure-treated wood.
Clean It Out - With any organization project, you'll want to remove everything to start fresh. This goes for your shed as well. Toss any old items or trash to declutter your space. If there anything you haven't used or your family has outgrown, now might be the time to donate or sell that item. Though removing everything from your shed might seem counterproductive, it's better to see what you have to store before you create new systems for it.