These ways sound simple, hence the title. But don't underestimate their power!
1) Reviews. And no, I don't just mean tonnes of positive reviews, but a mixed basket. A book with nothing but 5 star reviews looks fake. A book with mainly 4 and 5, but a few lower, looks genuine. So if you've just had your first 1 star review, don't worry. As long as the overall rating is above 3, you're fine. The lower ratings add legitimacy to the 5 stars!
2) An effective marketing campaign. Don't just promote willy-nilly, have a plan! If you don't your book will suffer. If you're unsure how to go about making a marketing campaign, check out my other blog: http://blogpostsforwriters.blogspot.com/2015/07/18-steps-to-successful-book-marketing.html
3) Teasers. Give away some of the good stuff! Find the most enticing few pages from your upcoming release, and make it visible online, and promote it. If the teaser is good enough it will compell pre-orders and downloads.
4) Give it away for FREE. By giving something away for free, readers can see if they like your book and writing style. More often than not, they will go on to buy your other releases, given that they are priced reasonably.
5) Low prices. All your books should be priced at $3 or less. Preferably less. No one would shed a tear over a couple of dollars on an ebook when they probably pay more for a takeout coffee. But if you price your book like the big players such as Stephen King or Dean Koontz ($20 - $30), you won't sell many. You can only do that when you have established a serious author prescence, and even then, you are isolating readers who can't afford it. I like to price my books low so any reader, regardless of their financial background, can enjoy my books.
6) Work with a blogger. Get a blogger to feature a review or teaser from your book. Preferably one with a large following in your target audience (genre).
7) Leave it to the experts. Use a promotional service, preferably one with a newsletter that goes out to lots of hungry readers!
8) Interact. Don't just mindlessly promote. Make it your mission to go into a group relative to your book's genre 'romance readers', 'horror readers' and interact with other authors and readers. And ONLY if the opportunity presents itself mention your books. But don't appear like a spammer or typical self-promoting author. The best way to do this is to mention something first about another book, then yours, then finish on theirs. THIS IS IMPORTANT! For example:
'I love that cover, it is fab. I can really feel the emotions of the book, and the colours are greatly used. It reminds me of the cover my designer made for my book. I bet that book is awesome, gonna have to add it to my to-read list!' And whenever you can, genuinely do add it to your to-read list as you should be in a group relative to the genre in which you write, therefore the genre in which you SHOULD read! Mainly, this is about having fun and interacting, don't go in with the mission of mentioning your book or it will be obvious and act as a reader-deterant.
9) Content Market. Write short stories, articles and so on, all relative to your book or genre, and promote them. This will seem less like marketing, and more like wanting to share an article for the readers.
10) Piggyback. Take a look at your favourite author, and see how they go about launching a new release. DON'T COPY, but rather get inspiration from them. Perhaps a book trailer, cool banner, advance reviews etc.