*check out the battle for "nerd credentials" at the site.

the main influence as far as I know was joseph campbell
and his work especially a hero with a thousand faces
a book that I'm eager to order
If your interested in star wars I would recommend learning more about mythology and the hero myths
hercules,alexander the great and even jesus
joseph campbells' work is what a lot of movie makers look to to see if an idea/script will work be successful and appeal to people

Added

The Hero with a Thousand Faces. Indeed, reading it helps as well, especially if you can trace Lucas' archetypes (and heroes from your favorite movies) back to the book