Chip Kidd has been doing book cover design for Knopf for about twenty-five years, and has done some awesome work. He discusses it, with illustrations, in this TED talk, which is well worth a watch, whether you do your own covers or hire other people to do them for you. Knowing what a good cover looks like, and what the possibilities are, is massively helpful when it's time to decide whether or not the person you're paying is doing a good job for you.
Note that Mr. Kidd has the unfortunately common Major Attitude toward e-books. [sigh] I wish people would just get over the whole, "But-but-but the smell of a book!!!" thing already. :P As someone who prefers paper books, it's embarassing how some folks who (unfortunately) share my preference get all sneering and snarky about it. Dude, it's a format. You're allowed to prefer whichever one you like. No reason to insult the other format, and by extension, all the customers who like it. I mean, seriously, do these people really think that if they just slather on the snark thick enough, often enough, the rest of the world will eventually smack its collective forehead and exclaim, "Wow, you're right! This whole e-book thing was a horrible idea! Let's just stop making them and go back to good old (smelly) paper!"
That one annoying quirk aside, Mr. Kidd is a incredibly talented designer. If you have anything to do with making book covers, whether putting them together yourself, or approving and paying for the work of others, give this a watch.