In "The Phantom Menace" Qui-Gon told Obi-Wan to be mindful of the Living Force. We have never heard any Jedi mention this in any of the other films. I think that Qui-Gon was a rogue Jedi who believed in a side of the Force that was not taught..a side that focused on the individual, which would explain why he wanted to train Anakin by any means he could. Believing this before he died, perhaps he, being possibly the only Jedi to believe in the Living Force and be mindful of it, died for all of the Jedi, which would probably explain why his body was burned and did not disappear. Maybe if a Jedi fully trusts the Living Force and puts it into action and focuses on the individual person that needs caring about, he or she will be able to come back to make contact with the living world. Qui-Gon taught Obi-Wan this, and that would explain why Obi-Wan is able to come back as a spectre and make his voice heard by the living world using an individual. Perhaps Yoda learned this too later on and practiced focusing more on the person's needs and not always the situation elsewhere.