h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

According to Robert Heinlein, longevity is something that can be bred for. If you ensure that only those whose grandparents lived to 100 have children, then those children should live longer.
According to Neil Gaiman, it is possible to live forever, simply by refusing to die. Apparently it is all in the mind, certainly of a character called Hob Gaddling (if I got the spelling right)

Another fictional -- or at least science-fictional which may/may not be the same thing -- is from Robert Heinlin's _The Moon is a Harsh Mistress_. The idea is to go to some planetoid -- in this case, the moon -- with a lower gravity than Earth's. Less wear and tear on bones and most, if not all, organs means a longer existence. Since the moon has approximately one-fourth the gravity of Earth, this means you could concievably last as long as two to three hundred years!

Of course, this is all dependant on 1) actually building some sort of habitat on the moon where the cosmic rays, lack of air, lack of water, lack of food, and wildly different temperatures won't kill you and 2) never, ever going back to Earth, because your body will be crushed by the "normal" gravity.

Another fictional long lived character can be found in Edgar Rice Burroughs stories about Barsoom (Mars).