Here, I will present information that I used to prepare for my trip and things I wish I had done / known. This is Chapter 1.
Call up the Cité Internationale des Arts and ask about housing. Do this as soon as you know that you are coming. If you are two late to apply for the start of your stay, apply to get housing for the second half. This housing is probably cheaper than what you would otherwise find and it gives you access to the art scene, including a performance space. It is really hard to get gigs in Paris and very few people have heard of CCMIX. Being in the Cité des Internationale Arts will help you out.
Learn as much French as you can. Take French classes at your school, at Alliance Française or in summer school. The 2-semesters-at-once summer class at UC Berkeley is the cheapest option for a Californian. Also, the teacher and TA will help you with your paperwork for a visa.
Get a visa. The school tells you not to worry about visas. None of their students have been deported ever. The anti-immigration hysteria is not about Americans. However, getting a visa may be worthwhile for a few reasons: 1. If you are age 26 or under, the French government will give you money. 2. If you ever want to return, you won't be faced with the choice of lying or admitting you've been in the country illegally. 3. The paperwork will help you buy a cell phone and open a bank account and things like that. You must go to the consulate in person to get a visa. Call them for more info. In the morning. It may be that you only need to appear in person once.
Tell everyone you know that you are coming to Paris for the year and then ask them if they know anybody there. Get phone numbers and email addresses. Call those people as soon as you arrive and arrange to have a drink with them or something. Meeting people in Paris is difficult and making friends takes a long time. Get started quickly.
If you want an apartment to yourself, Craig's List Paris can be a good place to find one, as can FUSAC. Having a housemate, however, will help you meet people and will be cheaper. If I were coming by myself and I didn't get a spot in the cité, I would stay in the CCMIX apartments and look for a housemate once I arrived.
If you like to read about things before doing them, French or Foe has good advice about cultural differences. The philosophy seems to be right, but unfortunately many of the specifics are outdated. The author is right when she says not to bring wine to a dinner party, but (probably) wrong when she says not to go to the bathroom at somebody else's house. I say "probably," assuming you're not going to be dining with any high government officials or anybody's formal grandmother.
This list is not exhaustive. Obviously, you must also do things like gather paperwork. If you think I left anything out, please leave it in the comments. More advice about other aspects is coming in later posts!