Going North to the Cévennes and Clermont-Ferrand
Except during summer time, that's the only way to get yourself into the beautiful national park of the Cévennes, so that's pretty easy to do, at lot of local people are hitchhiking. When you're go out of the train station, turn right and walk all the way to the river (le Gardon). Then turn right again and follow the river until you find the stop located on the hitchwiki map, after the high school with a weird architecture (don't worry, you'll directly see what I'm talking about). There is some place for cars to stop, and it's the main road to go in this direction. But keep in mind that it can be very quiet after 7:00pm.
Going South to Nîmes, etc
Follow the river in the other direction, and try to go on the N106. But there apparently isn't much place to hitchhike there.