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). These run down Loa Street. If you ask from where you catch a bus to Yotala you may be told you need to go to the Mercado Campesino. Those are the vans that also go to Yotala and may also pass you on Loa, though likely quite full. From Yotala's trance 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 OpenStreetMaps) 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 northern 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.