In a bookstore you almost always judge a book first by its cover.
A successful book cover performs a remarkable feat. It manages to silently capture a potential reader's eye so well that he must pull the book from hundreds of other books fighting just as hard for his attention. This is the critical first step toward a sale. Traditionally a publisher handled the marketing of cover design, but as self-publishing evolves many writers are now in control of all creative aspects of their book, including that all-important cover. From the colors you pick to the images you use, every choice you make has an important part to play.
Be Genre Specific
If you wander through the aisles of any bookstore you may notice that some books are marketed differently than other books based on the genre in which they were written. Many romance novels and "bodice-rippers" feature good-looking men and beautiful women in a heated embrace, to hint of the passion contained within the pages. The more skin shown, the steamier the story is likely to be. Likewise, mystery, crime fiction and horror all allude to the suspense and thrills the reader can expect once they open the book. Study the best-selling books within your chosen genre to see what works best for your particular story.
Make It Pop
Because your cover has only a limited time to catch a reader's attention, cluttering up the cover with a lot of details can actually work against you. Focus on simplicity to make it pop, using color to set an emotional tone. In the case of Stephenie Meyer's "Twilight," two pale young feminine hands hold a blood red apple against a stark black background. This successful use of simple symbolism alludes to the forbidden fruit of this darker paranormal love story, which is striking both as a book cover and a thumbnail photo alongside all its rivals. Additionally, books in a series tend to carry over similar themes to identify them as part of a series.
Be Professional
Your work is about to compete with books that were marketed by traditional publishers. Its cover has to prove it can stand side by side with "the big boys," because you have only an instant to convince the reader to spend his hard-earned money on your book. Everything from font choice to design placement counts. Self-publishing services like CreateSpace offer design guidance with a variety of templates you can use for a more professional edge, without the additional cost. Research what services are available through your publisher.
Considerations
Use caution when using photography for your book cover design. You may find the perfect photograph through an Internet search, but unless you have written consent to use the photograph for commercial purposes you can open yourself up to copyright infringement. There are services where you can use royalty-free stock photography, which means you pay once for the use and not by volume of books sold. Many cover-design templates through self-publishing offer limited photography you can use for your project. If you have the means and the creativity, you might even use your own photography for your cover design.