Ask yourself some important questions about the book. What kind of book is it? What genre is the book? Let the answers dictate the type of cover you're after. If the book is instructional, an air-brushed barbarian wielding a sword certainly will grab your attention, but will not be true to the content of the book and will mislead readers.
Once you know the themes and intent of the book, you will have a better understanding of how to approach the cover design.
Discuss ideas with the author or publisher. Most authors or publishers will have some ideas in mind already, and it's important --- especially in a collaborative process --- for everyone to be on the same page.
Research. Note what books get your attention and what books turn you off. Look at best-sellers and note which have dynamic cover designs. This will inspire inner creativity and guide you away from ideas that won't work for your project. While researching, assess the audience of the book. If it's a romance novel or a self-help book, see what similar books' covers look like. This could save you a lot of time later on and prevent you from going in the wrong direction.
Draw up some thumbnail sketches and make some rough drafts. Having haunted some websites and bookstores for samples, you should be ready to lay out some ideas, so make several drafts. Treat this as an organized brainstorming session and it will yield lots of positive results. Even negative results at this stage are constructive because you're learning what to avoid.
Find a strong image. Determine if you will use an illustration or a photograph for your cover and make sure it's one that will get people's attention. It should be derived either thematically from the content of the book or be an event, person, place or thing from the book. If the book is instructional or business-related, then maybe a graphic is not needed.
Select the right font for your cover. This may be the single most important factor in creating the cover of your book. Above all else, the book cover needs to easily be read. Consider a font that is eye-catching, but avoid using outdated fonts or overly ornamental fonts that are confusing to the eye. Lots of font samples are viewable online and sites like Dafont and 1001 Free Fonts.
Enjoy the creative process. In fashioning a book cover, delight in the act of design and creation. A happy frame of mind is fertile and productive and the best art can be conceived in this way.