Give yourself plenty of time to pack. It's very easy to leave it to the last minute, panic, and then realize you've either got too much or you have forgotten something vital.
One of the single most important things you will need in France is the ability to speak French. You are bound to need to ask directions or read a sign or a menu that is only in French. If you don't have a translator, you could truly regret it! You can try learning the language before you go, but you'll find that when faced with rapid-speaking French people, your mind will go blank.
What is particularly good about this particular translator is that it is a talking version, so you won't have to deal with blank stares when you mispronounce a word. It also features a currency and metric converter, calculator, alarm clock, calendar, voice memo recording, and eight travel games.
If you plan to plug in anything you bring with you, it won't fit in the holes unless you bring an adaptor (and these are surprisingly hard to find once you're in France, particularly in small towns), and it just might get burned out if you don't convert the electrical current. This is the best investment you can make: a few bucks to save (and be able to use) your electronics worth hundreds of dollars.
In France, you walk. You walk to shop. You walk along the Seine. You walk to attractions. And when you get to those attractions, you walk up 10 flights of spiral stairs or spend hours wandering through the galleries of museums. This is the best investment you can make before your trip. Get a nice pair of high-quality walking shoes. If you don't have the cash, at least get a couple pairs of walking inserts.
Avoid ear pain on the long flight to France with these wonderful, inexpensive Earplanes. You'll find them indispensable to avoid pesky popping and pain. Just stick them in your ears on the departure and for the descent. They even come in versions for children, which could make for a much more peaceful flight for everybody concerned.
The worst feeling is to be in France, lost, and only have a limited grasp of the language. Avoid such mishaps with a very good France map, which is absolutely crucial if you plan to drive at all. There are lots of versions, so check them out before you go to make sure you have the map that you can easily use.
For increased security in a foreign land, many people feel much better having an alarm. Wedge this under your hotel door or train compartment door and, if it opens, an alarm will sound. it just gives you extra peace of mind when traveling abroad.
This may not seem crucial, but this is worth investigating (especially if you are planning to visit Paris during the rainy/windy season, which some years can seem to run from January through December). It is also quite important to get a good umbrella. Everybody has had plenty of compact, travel umbrellas that bite the dust under Parisian winds. This one is sturdy and specially vented so the wind won't flip it inside out.
As anyone who has traveled around France by train knows, versatile luggage is crucial. The rolling tote is preferable because it can be carried on your shoulder or rolled through the airports or train stations when your shoulders feel like they are about to burn right off. This model is nice because it is compact enough to fit under the airplane seat, but also has side pockets to quickly reach passport and boarding pass.