Some of the toll fee's on the French highways are hefty. I know that you can avoid them by taking the routes nationales, where some are almost like highways. I don't mind traveling a little bit longer to save money, but the 6 hours to drive 150 km I drove today to save 20 Euro was too much. Is there an easy way to determine when it is worth taking the secondary roads or when it is better to just pay the "péage".
Whether escaping the toll sections is a good idea or not really depends on the place.
So-called "routes nationales" (equivalent to the US Routes versus the Interstates) can be narrow winding roads crossing every village in some areas, and close to freeway quality with multiple separated lanes in others.
For example :
The A65 from Bordeaux to Pau cuts down 1 hour on the equivalent trip using the standard road, although it is one of the most expensives in France. Also, there is the added confort of not having to slow down and care for pedestrians in every town, as well as not being stuck behind a slow truck with no place to pass it.
The A75, being part of the multiple possible ways to link Paris with the Meditterranean coast, is free from Clermont-Ferrand to the southern end. Also, there is a small toll to cross the Millau viaduct. In addition to the beautiful view, the bridge helps you save more than 1 hour driving down the valley and up on the other side.
Going down the A10 from Paris to Bordeaux, there is a way to cut down €20 on the toll while keeping the travel time efficient. Many people exit at Poitiers-Sud and continue on the N10 "nationale" which is a divided highway with nearly the same driving confort as on the A10.
Most toll sections in France use a distance approach, where you pick up a ticket upon entering and pay on exit. Do not lose your ticket or the most expensive possible toll will be requested. The tip in the answers above apply, where entering and exiting multiple times can save you some cash.
A few sections, tipically shorter, use simpler, flat-toll barriers placed regularly, known as "tirelires" (money banks), with coin baskets. These are more found in the far southwest to the Spanish border, and around Nice. In my experience, it is better to pay the toll because, if you switch to the "nationale", you may wander into a good gridlock, especially during summer.
Also, other tips to save money include:
Fill up in towns if possible. Gas is more expensive in freeway service area stations. The supermarket stations have better prices, especially Carrefour and Leclerc.
Rideshare! "Covoiturage" in French. This is easy using dedicated websites, where you can announce your trip and let riders join you. One of them is covoiturage.fr; you can set options such as ig you are ready to pick-up/deposit someone en-route, if you listen to music or not, if you accept smokers or not...
This may sound off-topic, but: Consider taking the train. Saver fares randomly appear, and, in most cases, you will end up paying less for your ticket than adding the cost of gas plus toll. In some cases, the train fare is the corresponding freeway toll alone, or less. Also, using high-speed lines, you can reach your destination quicker. If you need to drive once there, you can review the "auto-train" motorail (car on the train) option or combined rail fare + car rental packages.
You can see the rates from A to B on the French motorways website (enter the departure and destination cities in the box on the right). You'll get a map and a breakdown of the route. Beware of the route shown near cities, it tends to show unnecessary detours.
The route breakdown tells you where the tolls are, but not where you would need to exit and reenter the motorway to avoid them. A good paper map would show you where the tolls are. So should a good GPS.
That same website also has a flash map with links to the website of the company that manages each motorway. These sites usually have more detailed information, but it isn't always easy to use as you'll sometimes need to know the name of the rural municipality where the motorway entrance or the toll is located, and many of these sites are in French only.
Some sites let you find an itinerary that avoids tolls, for example Via Michelin.
Remember that if you take the secondary roads, you'll be saving fuel by driving slower but you'll also be driving more kilometers, assuming that you could do most of the trip on a motorway. It's difficult to have a general rule as to what would cost less.
Got the following trick through other channels. You can save up to 20% by exiting a toll route multiple times. Apparently, the fares are not calculated based on the kilometers driven, but on specific trajectories.
Example: if you drive from Lille to Paris, you pay 15.60. If you exit and immediately re-enter the highway three times inbetween you pay 11 Euro.
Someone made a very handy map to calculate your saving: http://www.autoroute-eco.fr/autoroute.html. (French)