I've been doing a lot of thinking about the question and realized that I can't just blog about promotion because I believe far more goes into a successful ebook than promotion alone. However, if I wrote everything I think works in one blog, you'd be reading a book. So I'm going to break it down into a series of blogs.
I've been amazed at the success of my books. They are simple stories, traditional, maybe even a bit old-fashioned. They are more "Americana," like Little House on the Prairie than "Western" like Lonesome Dove. And I think that's part of their appeal. There's a whole niche of readers who have missed "sweet" romances. They may have even stopped reading romance because they are uncomfortable with the sexuality. There's also a lot of other readers (like me) who just like romance. They won't care if it's sexy or if it's sweet. Readers who prefer "hot" romance will be able to tell from the cover and the description that these are not sexy books and, thus, won't buy them.
The other niche I've hit is historical Western. There are avid Western readers out there. Most enjoy both contemporary and historical Westerns. However, there aren't many historical Westerns published today, so fans of historicals tend to pounce on one when they come across it.
Therefore, if you're going to self-publish a book, think about your niche. It might be one that's tremendously popular--New York turns out lots of books in that subgenre. Or it might be a small niche, neglected by New York, that has avid readers. For example: Time Travel is a self-published subgenre that's doing very well for some friends of mine.
You might have to go with your instincts that readers will like your type of book. That's what I did when I believed that there were readers for sweet stories. The problem I always had was how can publishers find these readers if they've stopped buying romance? I've stopped worrying about that because those readers are finding me.
Or you might position your story to take advantage of a topic or activity that's already popular. Yesterday, I had a talk with a friend who's going to self-publish a book that was originally published in the late 1990s. The hero in the book is a retired Formula One race car driver. I suggested she make him a former Nascar driver. When the book was originally published Nascar wasn't as popular as it is now. Nascar has a huge and growing following. More and more women are are becoming fans. Therefore, she has a built-in market for her book.
Having a niche also ties in with your promotion efforts. Although I haven't done so, I've been thinking of promoting to the Christian market. My books aren't inspirational, but they do have some inspirational elements. I think that would also be a good word-of-mouth market. One woman likes the book, and she tells other women at her church about it.
I'm not saying write to the market or what you think the market is. I'm saying write the book you want to write. Just take some time to think about the possible niche or niches and see if there's anything you can do to increase your readership by tweeking your book in any way.