@Alison Loiselle, I rented out my attached garage to my condo in Cambridgeport for the last two years. Since it was such a popular area, I just did month to month and found a simple lease online that sounded good. I found a couple where one person was doing their residency and really wanted to know they had a parking spot when she came back at 3 AM and didn't have to dig out her car at 5 AM. They leased for over two years until I decided to rent my entire unit.
While I was renting, I helped my landlord in Cambridge fill his parking space, all people who worked at a company around the corner.
Craigslist worked wonders for both. Shoveling of the driveway was included in both cases. I also had to put down a lot of salt in the winter as there was a dip right in front of my garage door, causing water to pool and freeze. This wasn't a huge problem since I lived on-site.
Great experience overall, you might just want to clarify exactly where they are to park.