The main thing to watch out for is 'priorit� � droite'. You only need to worry about it when driving through built-up areas. 'Priorit� � droite' means that, when you're driving along a road, any vehicle entering that road, from your right, has priority. So you should expect to see drivers suddenly pulling out in front of you. (Or, better, you should approach all side roads, on your right, with caution). Note, however, that 'Priorit� � droite' is cancelled if there are yellow diamond signs along the main road.
The other thing which sometimes confuses British drivers is going round a roundabout. Basically, everything is exactly the same as in the UK but 'reflected in a mirror'. So, you go round roundabouts anti-clockwise, and give priority to vehicles already on the roundabout (which, of course, will be coming from your left).
Take care, as well, if another driver flashes his lights at you. The French do this to remind other drivers that they have the right of way, and NOT to indicate that you may proceed. (Actually, that's roughly what our own Highway Code says the signal means!).
If you're taking your own car (instead of hiring one in France), check that you're really confident that you can see every vehicle approaching, from the rear, on the left side of your vehicle, with no blind spots. It will be your left-hand mirror (together with your rear view mirror) which you'll need to check prior to overtaking. When I've taken my car to France, I've never been entirely happy about using the existing mirrors. I've purchased stick-on wide-view mirrors to add to my wing mirrors and an additional (suction cap) interior rear view mirror (which I've placed to the left of the existing mirror).
Also, see here:
http://www.day-tripper.net/drive-infrance.html
Chris