Finding storage space in a small bathroom isn’t easy. So my first suggestion is to limit what you keep in the bathroom to what really needs to be there in the first place. Move everything else out. Most of us don’t clean out the medicine cabinet or the vanity very often. You will be surprised how much room you can gain right there.
You may be able to bulk up on storage space with a tall, narrow box of stacked drawers; some come on wheels. IKEA has all sorts of space-saving storage units you might try. If you have open wall space, that’s prime territory for hanging storage boxes or rods from which you can hang wire racks. It may even be possible to cut into the wall between the studs and build in recessed shelving there.
If all of this seems overwhelming, you might want to hire a home organization specialist to evaluate your situation for you and offer custom solutions. See our slide show for all sorts of problem/solution scenarios.