Hi there,
Ive been to La paz and Uyuni. First of all, Uyuni is one of the coolest places ive been and you feel you are in another planet. by all means worth the tough ride.
1) From La Paz take a bus to ORURO (4hours).
2) From ORURO you take a terrible overnight bus to UYUNI (8hours).
**both La Paz and Oruro have big bus stations so it is easy cathing the buses.
3) You will get to UYUNI at 4am and right when you get off the bus various local tour agencies will be bugging you to offer their service. We went with one of them and slept in their offices for a couple of hours until the 8am tour started.
I only spent one day in Uyunu (full day tour @$25-30/pax). I then took the night bus back to Oruro/La Paz. There are several tours, 2days, 3days and more.
Be prepared to rough it as this is backpacking at its extreme. Very cold wather and few faciitiees. You can also hire packages from La Paz (by 4x4, small plane) but they are very expensive due to the remotness and logistics.
Enjoy and hope this helps.
Richard Leon
www.peruforless.com www.argentinaforless.com