Top 8 Cover Design Tips for Self-Publishers

"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."

"Reduce your cover design on screen to the size of a thumbnail on Amazon and see if you can read it. Can you make out what it’s about? If not, simplify."

http://www.thebookdesigner.com/2010/06/top-8-cover-design-tips-for-self-publishers/