(Originally written in 2007. I now live here as of 2014) 
NUMBER 1: Not everyone here speaks English!
Ignore anything to the contrary that others who've been here might  have told you. Even in Paris, I've found very few people with a working  ability to speak English. So PLEASE come here knowing at least a few  basic things in French to help you get around. The more the better.  Especially if you will be traveling anywhere out into the countryside,  which you really should, its absolutely amazing out there. Besides its  only fair. We always say of others coming to our land, "If you're gonna  be in America you need to speak English." So why shouldn't the French  demand the same of us. Like I said, it really is ONLY fair.
(2014 Update: Still true.) 
NUMBER 2: Peanut Butter is an exotic item in France.
This one is especially for you parents out  there who may have children, like mine, addicted to the stuff. If you  would like to track some down and neglect my advice, which is to take a  jar with you, you can try looking where the jam, honey, and nutella is  (which by the way they have more Nutella here on one shelf than I think  I've seen in my entire life) but more likely than not it will not be  there. So try looking in the "exotic foods" section. Oh yeah and don't  look for a "jar" look for a can. That's right, a tin can, bearing almost  no resemblance to what we in America know as peanut butter, inside or  out. People here say they hate peanut butter and from what I've tasted  of the local variety, I don't blame them.
(2014 Update: Not true any longer. Skippy has taken the nation for peanut butter lovers everywhere and the creamy variety can be found in small jars pretty much everywhere. Also Mexican and Asian foods are found now too, although they aren't very good outside of a genuine Asian or Latino community store.)
NUMBER 3: Have good shoes and socks
Even in the country you walk a lot and my  stupid socks plague me. I got new shoes before I left and they are  working great, but my socks suck and fall down into my shoes. Ugh! Oh  yeah, lay off the tennis-shoes. People don't wear them here, major  tourist give away. They are considered tacky and ugly when worn outside  of an actual tennis court.
(2014 Update: Somehow Tennis shoes are now a European fashion trend. Not sure if it will last forever but it is here now) 
NUMBER 4: Know their customs BEFORE you come.
This really should be number one. Even  more important than knowing the language you have to know their cultural  and societal norms more than anything. Do not come here thinking this  is a "white western" world. Pretend like your going to Japan or China  and learn a bit about the French's social protocol. Certain things we do  that are considered "rude" or "polite" are the exact opposite of each  other. For example! In America you do not greet people in stores or  restaurant. You are in there for business. If you are asked anything its  only to find out what you need. In France its expected that you  exchange greetings with the clerk or store owner before conducting  business. Not to do so will ensure rude horrible service because they  feel they are acting in kind to your behavior.  Their business is not  just business, its like an extension of their home and they expect that  you would politely greet them first. And this is just one of many.
The only other one that is really important  to know is that a waiter will not be giving you a lot of attention. Here  to be constantly going to a table that has not asked for your services  specifically is "interrupting" them. But an American would almost never  "hail" a waiter and would think that if they had to do so it is because  he was "ignoring" them. These things are a big deal here. Know them.
(2014 Update: Still VERY true) 
NUMBER 5: The French do not hate Americans.
Get over it people. We've been fed this  lie from our Franco-phobe government and it just simply is not true. The  French are a proud people, as are we, and two proud, strong people can,  more often than not, rub each other the wrong way. What the French do  hate is people who come here with no respect for them, their culture,  their country, or their language. Superior attitudes and objectionable  behavior bother them more than where you are from. Pretend like you are  going to stay at someone else's house before you come here. Learn the  rules of the house. And then respectfully obey them. You would want the  same done for you.
(2014 Update: This was written during the Bush/"freedom fries" era. Political context aside, still true.) 
NUMBER 6: Bring a lot of money.
Even in the off season its REALLY expensive. Even eating cheap  stuff from street vendors can still cost you about 10 euro ($15) a  meal. And where are all these creperies I was promised! I finally find  one of these apparitions and wouldn't you know it, I wasn't hungry.
(2014 Update: Still expensive here.) 
NUMBER 7: Prepare thyself for dehydration!
In  America we use REALLY big cups compared to the French. Seriously! The  cups in some places would look like buckets to them. Their large glass  looks like our small and their small glass is a dixie cup. So I would  highly recommend cutting back on your fluid intake a week or so before  you come. Or you could bring your own cup and give yourself away. :)
(2014 Update: Definitely still true.)
NUMBER 8: Fear not for you Big Mac attack.
The  French have McDonald's EVERYWHERE! It's actually a bit depressing. Even  way out in little towns in the middle of no where we saw McDonald's (The French call it McDo, pronounced, "Mack-Dough"). BUT  here is where we found out its a tad different. French law regulates its food better than we do. So the ingredients tend to be fresher and better tasting than it's US menu item equivalents. You will  still find some things different, even in McDonald's, like Espresso  instead of coffee, but no one hears me complaining about that one.
(2014 Update:Still true only now I see more of them and more highway signs advertising them than I had before.) 
NUMBER 9: We're sorry but number 9 is on strike at the moment and can offer no information at this time.
(2014 Update: Yes, SO TRUE!)
NUMBER 10: Prepare to be amazed.
Nothing  you would ever see in a picture, or book, or hear someone tell you  about this place could ever prepare you for the grandeur and artistry  that is "France." It is not just Paris in the same way New York does not equal "America". This entire place, every city,  house, tree, or blade of grass is just amazingly beautiful. I don't  think there is anywhere in the world quite like it. It's like you can't  find "ugly" here. Ornate, perhaps even gaudy, but never ugly. Everything  here is about striving to create beauty. The land itself seems to have  taken up the cause of its people by decorating for each season and  setting its ornaments all into place. Or perhaps the chicken came first  and because the land is that way the people couldn't let themselves be  out done. Who knows? but bring a lot of film and prepare to be  disappointed at the inability to "capture" something.
(2014 Update: Perhaps a bit overstated but generally true. And of course no one uses "film" in their digital cameras or smartphones.) 
