On Cover Design
Covers are a big sticking point for indie authors. It's a great shame that a good book can be completely spiked by a bad cover. Of course, the majors are not immune to such things either - see the controversy over the whitewashing of Magic Under Glass, for example - but that's more a matter of bad choices than bad graphic design. I see more of the latter than the former among the indies.
The Basics of a Good Cover
They're mostly the same as any other design. It should be clearly read at scale, informative, thematically appropriate, and compelling. Simple enough to say, but unfortunately not so easy in practice.
An author can develop a serviceable cover concept simply by keeping in mind where the book is going to be displayed. For ebooks, the only thing that matters is the size of the image on the screen. Consider this: a list of Amazon titles only allows an image 115 pixels in height for each book cover. On standard 19" monitor being sold today, that makes it about an inch high. On the title's page, it's a little better - about 220 pixels in height. This is still pitifully small in comparison to way that physical books are experienced.
A Case Study in Design
Recently I had the unfortunate task of informing an author that their cover was a complete disaster. The font was inappropriate, the colors were terrible, it didn't work at scale, and it was not thematically in line with the book. I had no doubts at all that this cover would have spiked the book rather badly, and here's why.
The font has to work for the book. A bold, sans-serif font will not work for a romance title, but it will work for a space adventure. In either case, it must be readable at small scale. An author should never, ever choose a fancy font that isn't instantly readable at the scale of an Amazon listing.
The colors have to fit. This really means that the colors have to be appropriately eye-catching and geared towards the target demographic. Men and women respond differently to color, believe it or not, and if you know your audience, you can tailor the colors to them.
Never forget scale! Having a cover that works at different scales is so very important. Take a look at the cover for The Hunger Games, for example. It works at small scale, and at a bigger scale, there's all this extra detail that makes it interesting.
Theme is important. What's on the cover should represent the majority of what's in the book. No, it's not appropriate to take one scene from the middle of the book and build a cover on that unless it represents the entire book, no matter how important that one scene is. (Say, for example, you wanted to make a cover for Pride and Prejudice - would you choose an image of Jane, sick and bedridden in Netherfield? No. It's an important scene, but the book isn't about someone being sick.)
Compelling to Readers
This is the oldest problem in advertising - how do you create an image that compels someone to investigate further? The cover is essentially an advertisement for the book. It functions in exactly the same way as any other advert. It needs to grab the attention of the reader and entice them to look a little longer, and read a little more.
All I can say is that you should simply look at the best advertisements, and see how they did it. The principles are the same. Look for cleverness, boldness, different layers and aspects. Think of the "big picture" themes of your book, and look at what iconography best represents them. If it's all about action, or all about romance, or all about horror, then look at other books in the same genre and see what the most popular ones are doing to get some inspiration.
Finally, you should not be tied to your covers - yes, you may love the look, but if it's pushing away your readers, then back to the drawing board you go.