I thought I had saved the link to the blog on cover design, but apparently I didn't. I searched the net, and couldn't find it. So. Here's my guide to cover design.
Use only three colors. There can be variety in the lightness and darkness, but stick to the three colors.
Use only two fonts. I have seen this in a couple places.
Have only one written focal point. The author name should not compete with the title. Neither should the series name or subtitle, if you have either one. Keep the title the main attraction.
Have only one visual focal point. That can be a cluster, say two people close together, but have only one visual focal point. And center it!
Draw the eye toward the center. You want folks to give the book attention, so be very careful what goes off the cover. Background stuff like mountains or trees can be chopped off, but not animals or people. Also, we tend to look where others are looking, so don't have anyone, or any animals, looking off to the side. Everyone should look toward the center, or out at the reader. This is true for pictures within the book, as well.
I think that's all the tips I have. At least at present. Hope this helps others.