Perpignan is a cozy town that connects France with Barcelona. There isn't much to see, but if you happen to end up there, it is well worth a visit. It is the first bigger town when entering France from Spain, and so a useful place to change vehicles.
Going South and North
When traveling North, it is recommended to wait by the traffic lights on Cours Lazare Escarguel where traffic occasionally stops. It is possible to get a lift from here to the autoroute ramp further up.
You might as well try walking to the autoroute which is a very long but nonetheless very nice morning activity (for some). Just follow the signs for Barcelona. You'll probably walk for hours through many roundabouts, and at one point you even have to go on a sort of nature walk (hitchhiker Zac can't remember what prompted this but there was an obvious reason and a clearly cut trail through the woods that passed under a bridge), and eventually you'll get to a long, lonely but beautiful road with viniards on either side. Keep going, and eventually you'll get to the autoroute onramp situated on a bridge over the A9. You can stop here if you want to try your luck, or you can go to the péage which is visible from the bridge. Just follow the road down there and cut into the woods to get around and come out on the south side of the péage. From there you can thumb a ride, and there's even a parking area big enough for a lorry to stop for you. Note: getting to the other side of the péage involves crossing a creek and jumping one or more fences! Attempt at your own peril (or pleasure!).
Another good spot to wait is a small roundabout 10 minutes north of the river on foot. Walk North on Cours Lazare Escarguel, cross the river, where CLE changes its name to Pont Arago. Just before the sidewalk ends, take the stairs (on the right) down to the street below. Continue walking north, parallel to Pont Arago for 5 - 10 minutes. The road will veer to the right and you will be able to see another roundabout. Cars going both North (Toulouse, Montpellier) and South (Barcelona) frequently pass by here. I've successfully hitched from here twice.
Other options to get to autoroute
To get to the peage (pay toll) by public bus, the best is to take the line number 3, direction Pascot. Get out on the last stop, then walk until the end of Avenue Pascot, then go left on Route de Canohes until you see the big road (road D900). Go on right and follow this road for few hundred metres untill the toll. From this toll you can easily get a lift North or South. There is an international fruit and vegetable wholesale market (Saint Charles Market) close to this toll, many trucks that come from there enter the autoroute through this toll to go both to Spain and France. If you go to Toulouse, it is best to use a sign and accept only lifts going there because 60 km North in Narbonne the motorway splits. Equally, if you want to go to Montpellier, don't accept lifts that don't bring you directly to your destination.
If you're going to Spain and your driver is only driving to Le Boulou (which is the last exit before Spain) avoid getting out on the last petrol station called Village Catalan; on the map it looks like the last best place to hitchhike but no one going to Spain stops here because petrol is cheaper on the other side of the border (as of 2008). Instead, go to Le Boulou but ask your driver to drop you off at the final barrières de péage where all the traffic will have to stop (not to mix it with a small toll barrier on the sliproad which is used only by locals to exit and leave the motorway).
Another handy thing to remember is that buses within the province only cost 1 euro no matter how far you go! That way you can get almost to Andorra's border for only 1 euro.