Create a free website. These days, the core of all online and in-person marketing tools is a website -- a place where potential buyers can go to find out more about you, the author, as well as your book. You don't need to have formally studied html to craft a nice website. Free website builders help even the novice computer user create a site that includes the ability to order and purchase books directly from the site.
Link to your audience. If your book is non-fiction, research websites that are frequented by readers interested in your topic. Websites that sell products related to your subject are of particular value -- they may be willing to sell your book through their site. At minimum, they may be willing to read your book and post a short review -- and a link to your website.
Add a book-related signature. Adding an automatically placed signature line about your book to all your outgoing email helps get the word out. You never know when your site will pique an email recipient's curiosity and prompt them to investigate your product further.
Use social media. Sign up for a Facebook and Twitter accounts and use them to get the word out about your self-published book. Start by friending your family, old friends from high school, co-workers, anyone you know. Join writer's groups or groups that discuss your book's topic on Facebook and friend as many of the other members as you can. If you are going to be at a book signing or local author reading at your neighborhood bookstore, post that information for everyone to see. Occasionally post reminders that your book is available and, in two sentences or less, explain why people should buy it.