A good book cover can make the difference between someone picking it up from the shelf or perhaps even choosing it on the Internet, or not. So if you're self publishing make sure you do your homework and get the best designer or image you can afford, or even one you can't afford!
You'll need a good relationship with your designer, one where you can communicate the style, genre, aims and objectives of your book and at the same time respect his or her skills.
The cover image needs to grab the reader's attention and convey the type, style and essence of the book.
Many people only buy a couple of books a year so they often play it safe and stick to what they know which is why when a certain book is successful you begin to see so many copying the style of the jacket image. Do your research. Is there a certain style that surrounds the type of book you've written? Are you able to emulate this?
An author's identity can be built through their book covers. Examine books from the same author to see what style has been adopted. If you intend to publish more than one in the same series then think this through to ensure you get the style correct first time and for future issues.
Also it is worth noting that the experts are seeing a trend towards starker, simpler covers, partly due to the importance of the Internet.This type of design works well online, where it's harder to see intricate or fussy design.