San Luis Potosí (city)
East towards Tampico/Ciudad Valles (la Huasteca Potosina)
The #25 bus towards Los Gomez (7 pesos as of 2013) takes you out of town. (The #11 bus towards Quintas might also get you there.) You'll pass a speedbump and then immediately there's a petrol station on your right. That's where you want to get off. There's another speedbump just past the petrol station, so you should be pretty good to either ask for rides directly or stick your thumb out.
South towards Querétaro/Ciudad de Mexico
Take Bus Nr. 22 at Almeda Juan Saberia/Avenida Universidad. It will go parallel to the highway 57 to the industrial area. Once you are on Avenida Industrial, the parallel street to the highway, wait until the bus turns right and go of the next stop. Walk 5 Min. to the higway and go on it and stick your thumb out. There is a nice bay where cars can stop and pick you up.
Another option is bus Nr. 24 which goes to Villa Posoz. You can check the bus routes here public buses/colectivos