Sucre is a city in Bolivia.
South towards Potosí
Take micro n°100 or 101 until Yotala (Bs.3), where you'll find a tranca (toll gate). From there it's pretty easy to get a ride, since cars have to slow down you can also talk to drivers. Alternatively, you can take bus n°4 (Bs. 1,50) and get off about 1-2 km after the tejar, which is a petrol station. From there you have to walk few minutes to get further out of the city, but then you can get a ride to Yotala and the tranca.
North towards Cochabamba
You can take a bus #4 (you can find it at Mercado Central, Mercado Campesino or just in Avenida Marcelo Quiroga Santa Cruz (it's better to find this street in Google maps - for now you can't find this name in OSM) which costs 2,5 Bs (about 0,36$) and go with it to the last stop - Cement Factory (Fabrica Nacional de Cemento) which is located at the north outskirts of the city just before Parque Cretácico. There is a good place to get a ride even directly to Cochabamba. You can stay just in front of the factory entrance but keep at mind that there is a lot of dust because of this factory.