At his awesome whatever blog, John Scalzi posted a short article about the best selling authors of 1912 and how they are basically unknown now.
Say what you want, the man can rock a tiara.
I agree with Scalzi that Writers sometimes (perhaps often) have delusions of permanence: we are -after all- still reading Shakespeare and probably will continue to do so for the conceivable future. But the reality is that we'll never hear of the vast majority of writers, even those with that achieved significant commercial or critical success in their lifetimes.
That being said, there does seem to be more surviving 'names' and 'titles' when it comes to speculative fiction in the last century. Is this just a temporary artifact because speculative fiction is still so new, or will a larger number of science fiction and fantasy authors will just simply have a longevity denied to nearly all modern 'traditional' writers? I don't have a clue, but it's interesting to think about.
Of course, for me the question immediately becomes what strategies could an author conceivably cultivate in order to help their legacy survive? Near as I can tell, authors should try and encourage others to build off, reference and remix their work to the greatest extent possible. Film and comic book adaptations probably contribute tremendously to our remembrance of genre writers. To whit, compared to the other 1912 authors, we'll probably never forget a relatively unknown (at the time) writer named H.P. Lovecraft. This is entirely because his mythos was 1) Awesome and original 2) Incredibly inspiring to other authors/artists and 3) He actively encouraged them to build off of his work. Writers interested in staying power could do a lot worse than following his example (with the writing/promoting bits anyway, the living with the aunts-social ineptitude-racism bits... not so much).