How to get from Salar de Coipasa to Sajama National Park by public transport?

Wikitravel describes the alternatives suggested by @pnuts:

Trans Sajama runs once daily shared van from Patacamaya to the village of Sajama (Bs. 25). It leaves north side of town in front of 'Restaurante Capitol' at around 1 pm. However, the van actually shows up around 10am, when people also start gathering, and it leaves once full. It is best to arrive early and get the front seat, which is much more comfortable and with great view. Return to Patacamaya from the village is at 6am (4am on Sunday). The ride is about three hours.

Patacamaya is in main corridor between La Paz and Oruro (and beyond), so you can catch any bus heading to/from La Paz and Oruro and ask the driver to drop you off at Patacamaya. There is also frequent direct shared minivan to/from La Paz (Bs. 10), which is fast and comfortable (quite new 8 passenger van with nice seats) and may be even cheaper than Oruro bus, as they probably try to charge you all the way to Oruro. Much slower (almost 3 hours) local bus to Patacamaya also leaves from Cementario area of La Paz (Bs.7).

Alternatively, you can catch a bus running several times a day between La Paz and Arica and get off at the entrance to the park. The village is 12km from the main highway. Transportation may be arranged from nearby border town of Tambo Quemado, where you find Trans Sajama office.