Assuming you've done all the things like writing a good book, getting a great cover, etc, I think it's important to just get out there and interact with people, whether online or in real life. Don't overtly try to sell them your book - just talk to people, respond to blog posts, and always have a link to your blog or book in your signature. If people think you're a good person with interesting things to say, they might just click on that link and consider buying your book.
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