Establish a main focus for your cover
Every book has a story, and you want your cover to reflect that one idea clearly .
There should be that one element of the design that takes control of the cover and demands your focus.

Joel Friedlander has a great analogy:
“You could think of your book cover like a billboard, trying to catch the attention of browsers as they speed by. Billboards usually have 6 words or less. You have to “get it” at 60 miles per hour, in 3 to 5 seconds. “
This is exactly what needs to happen with your book cover. Readers should be able to tell at a glance:
- Your genre
- The general idea of your story
- Sense the tone of the book
- Spark an emotion (love, fear, sadness, curiosity, etc)
A romance novel will have a different tone than a mystery novel, and a nonfiction book cover should appeal to your brain instead of an emotion.
Legibility of the title
This is still a highly debated aspect of a cover design. Some professionals say that you have to be able to read the title at thumbnail size, and others say it’s not necessary. Some say it’s not necessary because Amazon (and other online retailers) displays your title right next to the thumbnail of the cover so if the reader really wants to know the title, they can read it there. However, it never hurts to make sure the title is readable at thumbnail size because when you run ads for promotions, you want people to be able to read the title.
Keep it simple
This ties back to the first point made but a busy cover can look amateurish and can actually turn readers off from wanting to look at your book. An author may see the cover as a masterpiece because the hero, his best friend, the space ship, the mountains and the dog are all represented on the cover. But not one thing stands out so all a reader sees scrolling by is a busy cover because they can’t make out what is happening or what the story is about within those 2-3 seconds. Remember, you design a cover based on grabbing your readers attention. They are the ones who are going to buy your book.
Don’t go overboard on fonts and colors
The general rule of thumb is 2-3 fonts maximum. The whole design has to work as one and this includes the font choices. Typically, the title is one font style and the author name is another. Try to steer clear of using two serif fonts or two sans-serif fonts. They typically don’t compliment each other and can make a design look amateurish.
Limiting your colors can also be beneficial, unless you have a specific (and logical) reason to use the entire rainbow. There are quite a few color palette pickers online to help you choose colors if you’re not sure where to start. Another good idea is to research what different colors represent. I am constantly referring back to the color book I own: Color – Messages & Meanings: A PANTONE Color Resource by Leatrice Eiseman. I’m sure there are other great books but I’ve found this one quite helpful.
Make sure images have a purpose
If you’re going to include an image on your cover, make sure it communicates something to the reader about your story. If the image doesn’t tell a reader something related to the main image, then maybe it’s not worth having on there.
A professional book cover designer should already know these, but the more the author and book cover designer can be on the same page, the better the cover design will be in the end.