Many bathroom designs - especially older ones
Go Vertical: Hang tall skinny shelves on the wall. This conserves floor space and makes use of the upper wall space which is otherwise wasted.
Build in Shelves: For a rather simple bath remodeling project, create a set of built-in shelves between the studs in the bathroom wall. You can add storage on a blank section of wall or along one end of the bathtub.
Magnetic Mirror: Add steel to the back of the mirror to magnetize it. Then you can attach hooks or little magnetic cups to hold small items like washcloths, cotton balls, or hair clips.
Door: Paint the back of the door with magnetic paint to create a surface for magnetic hooks or a set of movable shelves and bins. Consider replacing the single hook on the back of the door with a multi-tiered towel rack or set of hooks.
Roll-Out Baskets: If you're planning a bathroom renovation, it's the perfect time to install roll-out baskets or shelves similar to those designed for kitchen cabinets. These are handy for corralling small items and provide more usable storage space, especially in the back of the cabinet.
Pull-Out Towel Rack: Got a skinny space where nothing will fit? Consider a skinny towel rack that you can roll in and out when needed. You can buy free-standing units or multi-tiered racks that attach to the wall.
Corner Rack: Make use of that wasted space in the corner! Put in a tall, skinny rack that is built to fit into a corner. Most of these are designed for kitchen use, but there's no reason you can't put them in the bathroom and stock them full of extra towels, bowls of toiletries, and other necessities.