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.
If the clutter on your property says one unit isn't enough, but your wallet disagrees, supplement your main unit with one of the less expensive, smaller models. There are three main types of such units, all of which go by descriptive names. "Corner sheds" are made to fit precisely into a corner and run about 3 feet tall by 3 feet wide by 2 feet deep. Meanwhile, either a "vertical shed" (5 x 3 x 4) or a "horizontal shed" (3 x 5 x 4) may be just the handy outdoor storage building to fill your need for additional space.