I was recently at a baby shower for my advisor, where I was conversing with a certain author whom I've met a few times around campus. (Exceptionally nice lady, BTW.)
Her advice about publishing (although I want you to know it was unsolicited, since I was honestly not trying to snark writing tips from the token author) was that fiction really doesn't sell. Non-fiction does.
Therefore, one should start with writing non-fiction, and then you can branch out into fiction once you're already published. (It seemed to make sense, when I thought about, for example, Michael Crichton, who knows how to write medical matters in a way that laymen understand.)
I asked, "So, is that how you did it? Did you start with non-fiction?" Sheepishly, she admitted that she had actually started right in with fiction, although she attributed her initial "break" to the luck of being in the right place at the right time.