Great promotion + great story + market interest at the time + plain old luck
Every author that comes for a consult with my company 3L Publishing (www.3LPublishing.com) generally hopes for a successful book. No one comes to me and says they want to fail. When I counsel people I always tell them that their mindset determines failure. If the one outcome is a national best seller and the dream of joining that 1% of elite authors who dominant the market then that's loftier than I can promise. Of course, as an author who doesn't aspire to be in that 1% dream slice of the pie? For the sake of not setting up false expectations, I always tell them that if the idea is for the very fact that they authored a book to create new opportunities then with all assurance I feel confident the book will absolutely reach that goal.
In the meantime, authors can either hire a publicist to help them promote their books (and you can contact me at 916-300-8012 or send an email to info@3LPublishing.com) or do the following promotional tactics on their own.
1. If you have an author website you can for one day (or frankly as many days as you prefer) run a special and offer a FREE download. This effort will help get as many copies out in the public eye as possible. You want to create awareness about your book. So the more readers who become your fans, the more your fans will talk about your book. Then you close down the free downloads and direct them to purchase your book off Amazon or your website. Although generally speaking most people feel more comfortable purchasing books off Amazon.
2. Giveaways to book reviewer sites. Many book reviewers like to do giveaways. Even if they don't review your book, they will be willing to run a giveaway contest. Again, the idea is to build your following. When you have a first book, this tactic lays the foundation for more sales on your second book. For a series you will find this especially useful.
3. Set up pre-sales on Amazon and/or your author website. You can actually have an Amazon top seller based strictly on the pre-sales of the book. This effort generates buzz before the book even releases.
4. Give copies to your local book club and ask them to post reviews on Amazon. Now if you don't want to give the book club copies, you can offer them copies at deeply discounted rates. Book club members will spread the word about your book, too. More buzz is good. You want to get people talking about your book.
5. Celebrity endorsements, and if you can get a celebrity endorsement to put on the back cover even better. They have these shows in LA where you can buy a table and give your books away to celebrities in exchange for them holding it up and taking a picture. People tend to believe celebrities (just the nature of the game).