Do you know and trust anyone who's paying for a storage unit? Can you offer one or two people the opportunity to store their stuff in an extra room for a bit less than they're paying for their units?
Can you use the space for a side business run out of the residence (making crafts, dehydrating fruit/jerky, storing inventory)? This can be tricky to do legally, since all kinds of state/local regulations could come into play depending on what the business is, plus lease restrictions. If you can pull it off, though, you could probably claim a tax deduction for the space, which effectively reduces the amount you'd need to earn from the business to break even compared to a roommate. The biggest caveat is that you must meet the IRS's definition of a business. In a nutshell, this means you must be profitable (or at least be able to show that you're trying really hard to be), and you must regularly spend time on the business (as little as 5 hrs/wk is enough). If this sounds appealing to you, run it by an accountant since I only half-know what I'm talking about here.
