Randy Ingermanson, author and creator of How to Write a Novel Using the Snowflake Method, also publishes a free monthly e-zine for writers. In the December '07 issue he has an interesting article on launching your novel by building an e-mail database (includes an interview with indy Christian SF publisher Jeff Gerke.) I have some reservations about the e-mail database approach to novel launching and marketing, as I've been around that block before, but Randy makes a good argument for it.
For an incentive to tempt a book buyer, it has to be effective, personal and yet non-intrusive. To quote from Randy's article: Your goal is get e-mail addresses of people who are genuinely interested in what you are writing and who are willing to receive e-mail from you whenever you write a new book. I can honestly say that I've never bought a book because I received an e-mail from the author, the author's publisher, the author's marketing team, or a chain bookseller. But then, I've never signed up for any that I've received; they came in the form of SPAM which at the most I glanced at before deleting.
As incentives go, I lean more toward providing online free, original content that is available nowhere else. What writers do best is write, so why not use the greatest asset we have? It's also the type of marketing/launching strategy that any writer can afford -- all you have to do is write the story, turn it into an e-book, and park it on the internet. If the content is made available in those arid stretches between novel releases, loyal readers are more likely to click on the download link. Keeping free content available also creates a stock of stories for potential new readers to check out before they make a purchase.
The next hurdle is to find an effective, non-intrusive way to make a writer's free story stock more widely available, and I'm working on that.
What author-generated incentive(s) convinces you to buy a book? If you've got any good examples, tell us about them in comments.