I've dedicated a number of posts and comments to the subject of author platform. See for yourself in the results of this Google search. Rather than sifting through all those posts though, I recently discovered a book that does a far better job of explaining the ins and outs of this important topic.
Stephanie Chandler wrote an excellent book called The Author's Guide to Building an Online Platform. The good news is that she covers every aspect of building your platform and, as the subtitle states, "Leveraging the Internet to Sell More Books." The bad news is that the book isn't available till 5/31. I got my hands on a pre-release review copy and found it to be extremely thorough and helpful. (Even though the book isn't available today you could certainly pre-order it now so that it will arrive as soon as it's published.)
The author has fantastic credentials: She opened her own bookstore in Sacramento back in 2003, is the founder of BusinessInfoGuide, an online resource center for entrepreneurs and she authored two books prior to this one. In short, she's seen the business from just about every angle. In addition to telling you what's likely to work best for you, she also shares some of the things that haven't worked for her (e.g., in-store book signings). The book includes insight from other authors as well. Stephanie did a nice job sprinkling in interviews with several authors to get their tips and techniques. Thanks to this book I discovered a few websites and resources I hadn't heard of before and I plan to research them in the coming days; I'll be sure to report back on the ones that stand out the most.
If you're looking for a one-stop resource for author platform and maximizing online sales, be sure to check this one out.