If you are renting, the amount of liberty you face with remodeling is limited. Big-ticket items like vanity replacements over pedestal sinks may be out of the question, but there are some do-it-yourself tricks within your lease agreement to achieve the maximum amount of bathroom storage.
Utilize the space over your toilet. Often this space is underutilized. Shallow shelves and cabinets leave enough room to easily access the toilet but function well enough for the extra bathroom necessities.
Additionally, hanging hooks behind the door are great for robes and clothing. If you are opting for a space to hang your wet towel, look into placing a towel bar for the needed support. Do not forget you have space above the door, a perfect spot for a shelf for all kinds of bathroom fixings. For supplementary shower storage, look for an organizer. You can also look to installing a medicine cabinet. It may intrude slightly into your already small bathroom, but will fit the extra necessities you have.
Ultimately, the best way to get the most space for your not so big bathroom is to use less stuff. Really, that is the first rule of organization in general.