When I published my first Kindle book, I designed the cover myself. I was so excited to publish my book that I just used what I knew about photoshop and threw a design together. Later, I had the design revamped by a professional thinking I would make more sales, but to my surprise, my original cover and the elements I inserted on the design actually converted better.
Here are 4 elements I included on my original cover that converted higher than my revamped cover (that did not include these elements) and my thoughts about why they may be so effective:
A dark background - I think because it pops more. Lighter backgrounds kind of blend into Amazon's white background, even if you have a border around the cover.
A human face looking directly at the reader smiling - Human faces give trust, especially when the face is looking directly into your eyes. You will see many TV commercials or print ads using human faces. It gives your book a more personal feel and really connects with readers.
Author name is in large visible text - This is probably a branding issue. When readers see this, they may automatically associate your name with being recognizable and trustworthy.
Seals of accomplishment - When my first book hit the best seller list, I put a little seal on it stating so. If your book, or you as an expert, has earned any kind of recognition, your book's cover is the perfect place to toot your own horn!
Since then, I have done some testing on other books, and the outcome seems to be proof that including the elements above works best. (At least for me in my genres)
I would be interested in know what has worked for others. If you have done any book cover testing, and discovered similar results across the board with your books, let us know!