San Cristóbal de las Casas
San Cristóbal de las Casas is a touristy place in Chiapas, Mexico. It's a nice place to hang around, not too warm and touristy but definitely not as bad as say Playa del Carmen. It is known for being a center of indigenous culture.
West toward Tuxtla
For 6 pesos, you can take the colectivo R14 from 5 de Mayo near the zócalo to the autopista (highway). There's a stop light right at the entrance to the highway. Alternatively, If you walk on 100m or so, you will see the exit to the right that leads to the ring road/bypass around San Cristobal and also the toll-free (but winding road) to Tuxtla. Stand and hitch on the toll highway to Tuxtla just after the exit; it will be better for a fast lift to Tuxtla as you will cut out all the local traffic which will be taking the exit. There is a hard shoulder here for cars to pull over.
guaka, Sitarane, a Mexican girl and a German guy tried hitching out together but then split up. The 2 guys didn't manage to get a ride and finally took a local bus after waiting an hour or so. The girl/boy combo was more lucky and hitched a ride in less than 10 minutes.
From the bus station (terminal de autobuses), take a colectivo or walk east 4kms along highway 190 to the intersection with the southern ring road/bypass (Periferico sur). Just past the police post here is a soft shoulder/lay-by where you can hitch and where cars can pull off the road to stop.
Alternatively, you can also just walk to the terminal de pasajeros del sur, about 20 minutes from the center, and hitch from there. There isn't really a great spot to pull over, but Zenit caught a ride there twice without too many problems, for three people each time.
There are many hills around San Cristobal where you could find places to wildcamp. Be aware that tourists do occasionally get robbed in San Cristobal, so use your judgment and consider bringing a friend.