Study the best-selling books within your chosen genre to see what works best for your particular story. If you wander through the aisles of any bookstore, you may notice that some books are marketed differently than others based on the genre in which they are 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 and horror all allude to the suspense and thrills the reader can expect once they open the book.
Because your cover has only a limited time to catch a reader's attention, cluttering it up 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. The successful use of symbolism alludes to the forbidden fruit within this dark paranormal love story, which is striking both as a book cover and as a thumbnail photo. Additionally, books in a series tend to use similar themes, images and elements on their covers that identify them as part of a series.
Your work will compete with books marketed by traditional publishers. The cover must convince a reader to buy the book with very little else to go on. Everything from font choice to design placement counts. Self-publishing services like CreateSpace, Lightning Source and Lulu offer design guidance with a variety of templates for a more professional edge without the additional cost. Research what services are available through your publisher.
Use caution when choosing 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. Royalty-free stock photography services allow your to pay once to use a photograph, rather than by the number of books sold. Many cover-design templates also offer limited options. If you have the means and the creativity, you might even use your own photography for your cover design.