If you don't have GPS, bring a good road map. The rental car companies give you poor maps, if they give you a map at all. The best is the Michelin Road Atlas.
If you're going to rent a car, do it from your country. Renting a car in France is heart-stoppingly expensive.
If you're coming in summer, make sure you stipulate air-conditioning.
If you don't know how to drive a stick shift, ask for an automatic.
Americans, please note: European cars are smaller than US ones in the same category. Reserve a car that's bigger than you think you'll need. You won't regret it, especially if you have lots of luggage, kids, family or all of the above.
If you're traveling in a group, consider a mini-van.
For Americans, the price of gasoline is higher in France. Make sure your travel budget takes this into account.
In France, diesel costs less. So think about renting a diesel car.
Avoid filling up at motorway service stations. Instead, buy gas at hypermarkets or supermarkets.
When buying gas, double-check what kind of gas your car uses. Don't assume it's unleaded, it might be diesel. In French, unleaded is "sans plomb", while diesel can be "diesel", "gasoil" or "gazole".
When you pick up your car, spend 10 minutes figuring out how it works and where all the buttons are. For example in French cars, the horn is activated by pushing the left-hand stalk towards the steering column.
If you plan on doing lots of driving, make sure you budget for highway tolls. The longer the distance, the higher the toll. Some trips can cost upwards of $30.
Don’t tip in restaurants. The service charge is already included.
When it comes to convenience and customer service, France is different. For example, stores aren't open 24/7. Buying gas off the beaten track on a Sunday is a challenge. Don't try confirming your flight at 10 PM at night. Stores are often closed Monday morning, if not all day long (see next point). Some museums are closed midweek. In general, the pace of life is slower -- not always by choice, rather by law.
If you're going home at the beginning of the week, don't leave your shopping until Sunday or Monday. Outside of Paris and other major cities, you won't find any shops open on these days.
If your eyesight isn't what it used to be, bring a pocket magnifying glass. It'll make map reading easier. If you're visiting cathedrals, bring binoculars.
If you like wine and picnics, bring a corkscrew.