Friends ask me this all the time. Here’s today’s answer:

To the degree that I have been successful, it’s because I have developed expertise within a niche (women’s sports), then, because of my own passion for that subject, I show up (speeches, conferences, etc) where people interested in that subject congregate. Social media makes that much easier than the old days when you had to get on airplanes… but it’s a much more crowded space now too.

 
Whether in person or online or (ideally) both, the point of this approach is not to think of it as promoting a book, but as sharing a message. 
 
So you “go” to the places where those interested in that message spend time. And you continuously re-establish your expertise by blogging, tweeting, or writing a newsletter or articles so when someone thinks of that topic, they think of you. By contributing a steady stream of ideas, advice, insights, research, and commentary, you become known as a thought-leader on that topic, rather than the author of a single book.
 
Of course, you also identify yourself as the author of that (latest) book, so that if they’re interested in the subject of the article/blog/speech, they’ll be curious about (and MIGHT even buy!) that book.
 
Photo: I’m on the right (easy to spot in EVERY photo ) with good friends and authors Sam Horn and Maggie Bedrosian, at a National Speakers Association event.