In the prequels we learned that Qui-Gon Jinn was the first of the modern Jedi to accomplish this, at least partially. Qui-Gon didn't have enough time to learn the skill to appear as a spirit, but he was able to keep his conciousness in order to communicate with Yoda and with Obi-Wan. This is what Qui-Gon said about this skill in the script of 'Episode III': "The ability to defy oblivion can be achieved, but only for oneself. It was accomplished by a Shaman of the Whills. It is a state acquired through compassion, not greed."
Because of this, it seems to me like it's impossible for a Sith to use this very same skill, since a Sith will always act out of greed and self preservation. Darth Plagueis was able to extend life, but he certainly didn't use this particular skill, since he was able to influence Midi-chlorians. I believe that Darth Maul may have used a Sith version of the 'Jedi Ghost' to stay alive after his 'death' on Naboo.
Jedi Ghost / Spirit:
- Act out of compassion and sacrifice their lives
- Spirit has been kept
- Physical body is 'destroyed' and cannot be used anymore ("If you choose to face Vader, you must do it alone. I cannot interfere" Obi-Wan Kenobi)
- Based upon the power of the spirit
Sith:
- Act out of greed to stay alive
- Spirit is completely broken (= mad)
- Physical body can still be used, even if it has received the most horrible injuries
- Based upon the power of the physical body
The skill Maul could have used was based on his control of pain. Maul could sustain a massive amount of pain. Maul wanted to stay alive, no matter what the consequences would be. So in order to keep his body functioning, he gave up his spirit and totally lost his mind and memory while doing so.
The skill Yoda and Obi-Wan used (as did the Father on Mortis) was based on a conscious decision. They all chose to die (Yoda could have died earlier, but he waited for Luke) and they didn't die out of greed. They were able to keep their mind, but lost their physical body in the process.
So the Jedi skill was based on serenity of the spirit, while the Sith skill was based on the raw prowess of the physical pain (body).
