Step 1: Pick up wire paper trays from an office supply store; you can also find cheap vintage trays on eBay.
Step 2: Use them for storing bath and hand towels; stack 'em or hang multiples to maximize space.
Step 3: Let your friends think you bought them at Restoration Hardware.
Step 4: Put the aforementioned shampoo bottle collection in your towel closet. No one has to know.
Wire fruit baskets: cool for holding apples and oranges; amazing for holding bath products. Hang one from a shower rod or in the corner of your bathroom. Double the amount of storage space by hanging two together.
Lip balms, hair ties, bobby pins, bath salts -- all the annoyingly small goodies littering your bathroom drawers can be corralled in this wall-mounted spice rack. This homemade version and many others like it are available on (surprise!) Etsy.
Nail a couple hooks to your wall -- or, easier yet, buy a set of over-the-door towel hooks -- and hang a pretty basket on them. You get as much horizontal space as you would with a small shelf, and you don't even have to waste a half hour looking for your level and measuring tape.