Publishers want to know what your platform is, by which they mean how you will reach potential buyers for your book if they publish it. It's a foreign concept for a lot of writers, so here are some tips:
1. Understand this is not an overnight process. That's why it's a good idea to start working on your platform at least six months before the book comes out, but the longer the better.
2. Create a website that is relevant to your book. If the book is funny, make the site funny. If it's a horror novel, make the site creepy or about other works of horror--e.g., reviews, observations, recommendations, interviews. You need to attract the kind of people who are into your genre. Use WordPress, it has made having a site and blog easy even for non-techies.
3. Hang out on forums and sites relevant to your genre, and take part. Let people get to know you before you have something to sell. Otherwise you will hit the sites selling and that can turn people off. But once they know you, they'll be glad to hear about your book when it comes out.
4. Cultivate your local booksellers. Sadly, the number of independent bookshops is shrinking all the time, but there are still quite a few around. Buy at least some of your books there and chat with the employees and the manager (and in those small bookshops, often the owner is the manager and the clerk). When your book comes out, they'll be much more receptive to your proposal to put some in the window, have a signing or talk, etc.
That's part one--more to come soon!
(There are chapters on how to market your writing in my book, "Your Writing Coach," published by Nicholas Brealey--hey, why not ask your local independent to order it for you? More information at www.YourWritingCoach.com.)