When planning on working away from home, the search for housing needs to happen as far in advance as possible. This is particularly true if you'll be working in a city with a tight housing market (e.g., New York, Washington, D.C.). Because furnished housing is the ideal option for most, start your search with your network contacts. Ask family, friends, and acquaintances (dorm mates, classmates, co-workers, etc.) if they know someone who lives in the geographical area you'll be working. This is good way to find an apartment to sublet or a room to rent.
You should also check with your contact at the company or organization where you'll be working. These individuals generally know where past summer employees have found housing and can be an invaluable resource.