... I can, with confidence, disclose the results of my research: it's fairy dust.
Of course, it helps for your book to be well-written and gripping, and preferably one of a series. An enticing cover and blurb won't hurt. Many writers are enjoying modest success, making money and pleasing readers; but what is that extra something that fires people to tell their friends, post on Facebook or start forum discussions on Amazon? What makes people so obsessed with your characters that they write fanfic about them? I don't know, and interestingly, neither do the authors it happens to.
Take Hugh Howey, the latest indie phenomenon. He didn't promote Wool at all. It started life as a short story, published on KDP and left to twiddle its thumbs while Hugh concentrated on selling his full length novels. The short story sold surprisingly well, and eager readers asked for more. The rest is history. Hugh is a shrewd guy, able to work out what he wants and hold out for it; he also comes across as both nice and engaging, with a good relationship with his fans. But he admits he never expected Wool's success. I can remember him saying a year or so ago on Kboards that he was doing quite well, but wasn't in Amanda Hocking's league. He probably is now.
You can read Hugh's analysis here. This is how he begins:
I wonder if lottery winners get emails asking for advice on how to win the next one [...] Every week, I get a handful of emails from aspiring authors asking for advice. They want to know how I found success with my writing, and I find myself admitting that luck played the biggest part.
For more on this topic, read Hugh Howey and the Bestseller Myth.