Book cover design is comprised of text and images. In order to get the layout right, you need to think about the single message you want your cover design to communicate. What is the one feeling or idea you want to convey? Whatever message you decide to feature, make sure every element from color and image to typeface and text supports it.
Once you know your message, think about how you can best convey it. If you want to set a romantic tone, perhaps the main design element should be an image with double meaning. If you’re going for mystery, you might want your clever title to dominate, and a vague, shadowy image to support (but not overshadow it). Or perhaps you’re an author with a brand name, and you really want people to know this is a book by you. Then make your byline the biggest.