This week Joe Konrath wrote a blog post in which he talks about what things increase ebook sales and what things don't. Highly recommended.
One of Joe's most surprising points was that, in his experience, advertising doesn't have any significant impact on ebook sales. Good to know.
His post reminded me of Asimov's first rule of writing: writer's write. I know, that is about writing, not sales, but it's something I try and remember every day.