How not to end up out on the curb in Paris:
1) Wake up really, really early on Thursday morning and buy the PAP, a magazine with listings for apartments of all sizes and prices. This entails you calling the people who want to rent their apartments before 8:30 am. Why? Because you must devote the entire day to seeing apartments before someone else gets there. By 8:30 pm, all of the apartments listed will be gone. I promise.
2) Use all manner of websites, like apartager.com, to find an apartment or colocation. This is a good option, but you will most likely have to be in Paris to actually get something. Remember that when you go see apartments, you will need to have the full amount of first month's rent and the security deposit- either in check form or in cash, because otherwise someone else will have the full amount and they will get the apartment. Colocations are really cool because you get to live with awesome people and pay less for a bigger space (like a bedroom for you and a kitchen and bathroom for the apartment, instead of just have two burners in a room the way you would in a studio).
3) Use an agency. Never give the agency any money before you sign your lease. A lot of agencies are set up in Paris as a ploy to take your money and not give you what you want. I recommend Paris Academic Rentals, an agency that Sarah Lawrence, Columbia University, and Trinity College have contracted to help their students find housing. They charge a month's rent, but all of the places they have are clean, safe, and comfortable. If you want more info, just shoot me an email.
4) Go to Sciences Po's main hall (13, rue Saint Guillaume) and check the bulletin board for people looking for someone to fill their colocation. You could also leave your contact information on the board and hope someone contacts you.
5) Do all of the above in some order that makes sense to you. I ended up using the agency and found it lovely and very helpful. They also take really good care of everything and speak English which is useful if you don't.
Coming soon... The next step- insurance, dossiers, and cheap transportation.