I am really into manga (Japanese style comic books) and collect them the most. When I think of an idea for a story, I think of it as if I am either reading a manga or watching an anime. It was the first time I noticed that cartoons could be more versatile in their storytelling. So sometimes my ideas come from that area, or from a movie, but I do try to twist it to the point where it does become an original work. One big influence on me lately has been Gnostic Creation, which I think is a much better story than the one I've been taught.
I have found that listening to a certain kind of music helps, too. Celtic for fantasy and Ambient and Space music for science-fiction, for example. Also, and this is true, daydreaming can be very helpful to the creative process.