The Salar de Uyuni is not easily done as a day trip from La Paz, despite what some tour operators imply. The town of Uyuni is roughly 500 kilometres south of La Paz: an overnight bus takes about ten hours and arrives at dawn; a 45-minute flight (BoA or Amaszonas) is faster but more expensive. Plan on at least two nights: one night in Uyuni town (which has little to recommend it beyond being the gateway), and one night either in a salt hotel on the flat itself or in a basic guesthouse in the villages nearby.
Most people do the Salar as part of a three-day four-wheel-drive tour that continues south into the Eduardo Avaroa Andean Fauna National Reserve, where the landscape gets stranger: red and green lagoons at high altitude, flamingoes, geysers at sunrise, and the Salvador Dalí Desert (named for the surrealist painter, whose work it resembles). The Salar itself is best visited in the wet season (December through March), when the thin film of water creates the mirror effect that fills every photograph. In the dry season (April through November) it is a vast white crust under an enormous sky, which is a different kind of impressive. November and December, at the boundary of the seasons, can be the best of both — enough water for reflection, not enough to make driving difficult. Book a tour through a reputable operator in La Paz or Uyuni; four-wheel-drive vehicles and guides are mandatory.