Good reviews is my fourth reason. If Publishers Weekly and a dozen other magazines and ezines give your book a big thumbs up, you may be moving toward best-sellerdom.
This includes reviews on Amazon.com, the Barnes and Noble site—any place where people review or critique books. One example was David Wroblewski's debut novel, The Story of Edgar Sawtelle, which Amazon.com championed. The 566-page literary novel came out in 2008 and within a week after publication, it had gone into its seventh printing—a reported total of 90,000 copies.
That's not the only time Amazon.com has "made" a best seller. They listed Wrobleswki's book as their favorite for the month. Their other favorites aren't always that successful.
My all-time amazing example is Tom Clancy. In 1984, the Naval Institute Press published his novel, The Hunt for Red October. It's what I call a techno-thriller and reviewers praised it. The good reviews propelled it forward and it stayed on the New York Times' list for 25 weeks. Ever since, Clancy has been a brand.