First of all, lets clarify the term "immortal". If you investigate the viewpoint of various world religions and spiritual systems you will notice a range of immortality types described as being achievable by humans. Some traditions actually have lists of immortal types. These are typically found in the Orient. They usually boil down to these three basic types:
An immortal ghost would occur when the soul (personality or ego) of a person is able to survive the death of their physical body and subsist in this disembodied state for much longer than usual. Immortal ghosts are usually described as subsisting on the emotional energy of the living. This is what allows them to prevent dissolution in the second death. They also tend to anchor themselves to something physical, such as their corpse or grave, the place in which they died or a home they had while living. These types of immortals are described as vampires, haunting ghosts and hungry ghosts. Tradition states that this most often happens as the result of intense unresolved trauma either during life or as a cause of death. In most cases this type is undesirable.
Extreme physical longevity is pretty much just what it sounds like - having a much longer than average life. The "lower" end of the spectrum includes the slowing down of aging and improved resistance to illness. Someone who lives to 150 years old can safely be placed in this category. The other end of the spectrum reaches all the way to the complete cessation of aging and illness and a potential physical lifespan of several thousand years. Someone in this category would be at any point along this spectrum. Although much reduced, chance of death still exists at all levels within this type, especially from accidental or otherwise untimely causes.
The True Immortal no longer experiences aging, sickness or even injury. They have a perfect and incorruptible body that doesn't require food, sleep, shelter or even air. They can participate in the physical but are not bound by the standard limitations. The True Immortal can also travel bodily through all realms, whether physical or spiritual (a division which no longer really exists for them.)
I personally think that these three basic types of immortality are all possible.
Immortal Ghosts probably happen all the time. Folk wisdom tells us that when a loved one dies it is in everyone's best interest not to spend too much time grieving. The reason given is that extended intense emotional grieving feeds the ghost of the loved one, keeping them earthbound and delaying their "moving on." I don't imagine that there is a lot of value in this form of "immortality." Seems to me like it would be more of a form of bondage. It means that you would be a slave to your fear of dissolution. You would be addicted to invoking intense emotions in the living. I can't imagine this being a very satisfying state to be in. I imagine the ghost would likely not even be aware of their state, but would be like a person in a non-lucid dream. This is backed up by anecdotal evidence showing that many hauntings are done by ghosts constantly reliving their own violent death. In general, I judge this type of immortality probable but undesirable.
Physical longevity is something wished for by most people who are passionately engaged in life. There is so much to accomplish and so little time to do it. A lot of time gets lost when death occurs. For one thing, lets be honest here: nobody really knows what happens when you die. Also, even if reincarnation occurs, starting from birth means you will lose at least 15 years time simply learning how to survive. Not only that, but the overwhelming majority of people do not have conscious access to the memories of any past incarnations that their soul may have experienced except possibly in fragments or through an unexplained aptitude in some field. You'll still have to learn everything all over again - although it might be a bit easier this time. Now imagine if Albert Einstein, Nicola Tesla, Beethoven, Picasso or Nietzsche had even 25 more years of healthy life available to them. Just think of what any one of them could accomplish in that time. Now imagine if they had an additional one or two hundred years, or more!
Obviously true immortality would be the ultimate goal. However, I believe that it would be the very rare individual that would be able to accomplish this within a single standard lifespan. Just to come up with a ballpark figure, I bet that for most people a few hundred years of steady work uninterrupted by physical death would be necessary to make the jump from longevity to true immortality.
Personally, I think the best action plan would be to hedge your bets. That means that while working to unravel the secrets of true immortality you should also make efforts to transform your lifestyle into something that promotes a long, healthy life this time around. To start you off, I would recommend ordering The Healthy Brain Kit. This resource contains two audio CD's, a portable book and a set of exercise cards. It's put together by two medical doctors and it gives you info on diet and dietary supplements, physical activity, games and puzzles that build brain power, memory skills training, self-hypnosis, etc... Several of the exercises happen to be useful for developing the foundational skills needed to perform successful occult work as well.
Click Here to watch a video called "Exploring Life Extension."