Relationship with students: This is for you to determine based on yourself and your level of students(lycée, collège, etc). What I tried to do was to set the distance between myself and my students at the outset, then gradually move towards them as I felt comfortable. The opposite(starting as their friend then trying to distance yourself) is much harder. I accomplished this mostly by simply dressing like a teacher and acting like a teacher. Then, as I got to know my students, I ate lunch with some of them and developed friendships with a few. With younger students, they could be intimidated by this approach and less comfortable participating in class.
Carte d'Attestation Professionnelle: This is a card that you get from the secretary to the principal(headmaster) that shows that you are a teacher. This card will get you discounts on many things. For example, with this card you get into some museums like the Louvre and Georges Pompidou Center free. At other places, like Versailles, you get in at a greatly discounted rate(25FF). I've heard also that you get discounts on books in bookstores, though I haven't tried this myself.
Transport: For those in small towns(and even bigger towns) this can be a problem. Ideally, you have your own car. Otherwise, you're dependant on the train/bus system, which is rather good, to get from town to town or across large cities. Those in smaller towns rely more on their feet. A good idea is to get yourself french friends. Not only will they make interesting friends, they might have a car.
Cultural Differences: There are a few things(some unpleasant) that you'll have to get used to while living in France. These can include the pleasant things(bread, pastries, food in general, wine) as well as the unpleasant things(dogs and their owners who refuse to clean up after them, smokers aplenty, traffic, cell phones ringing in cinemas, stores closed at the most inconvenient times). All in all, you have to accept that you're in a different country that has some advantages and disadvantages. A good pastime like sports can provide an outlet for the frustration.
Travel: One of the most important things you can do is to travel. You'll have plenty of free time, especially holidays, to travel. Instead of returning home for every break, think about travelling. Also, the weekends are a great time to visit another city. Just call the assistant there, invite them out for a drink, and ask if you can stay on their floor for the night. Meeting other assistants was by far the most important and rewarding thing I did during the experience.
Time: Time for you will go by very, very fast. At first you won't believe me. October, November, and December are difficult because the weather is not so great and you're still getting used to the foreign country. Starting around January or February, time will fly by. Make the most of it!
Taxes: The following applies to american citizens working in France: If your income for a given year totals over $7200(that applied for the year 2000, it can change from year to year), you have to file an income tax form. However, if you live in France for more than 330 days of a given year, you qualify for a foreign earned income exclusion. Check out the following link to get all the details: www.irs.gov/tax_edu/faq/faq13-7.html And thanks to Beth for the info!