You might think about your cover as a promotional tool, but it is. Your cover is the first (and probably only) ad that your reader will see for it. It will be used in any promotion you do--guest blogs, interviews, etc. So make sure it's eye-catching and appealing.
One of the oldest writers' laments was "My book has a terrible cover!" But for self-pubbers, the ball's in your court. You can make or commission a cover that's perfectly suited to your book. But what constitutes a good book cover? If you're not an artist, how do you go about hiring someone for one of the most important decisions of your book's publication?
Look at books. Seriously, walk in a bookstore and look around. Which covers grab your attention? What makes them stand out? How is the text arranged? What colors do they use (or don't use)? Are they simple or complex? And how are covers designed within your genre? You don't want to copy everyone else, but you do want your book to fit in and give a quick visual clue as to what it's about. For romance novels, a cover with a photo of lovers in a steamy embrace is normal; for a book on economics, not so much.
Decide what you want. I sketched my cover in pencil to get a general idea of what I wanted, well before I hired an artist. However, if you can't think of anything, an artist will probably be able to come up with ideas for you. Be prepared to have a description of the book or characters ready if you want a portrayal of one of these on the cover.
Get a budget. A full, hardcover print layout could cost a thousand dollars; a simple ebook cover might be commissioned for under $100 by an experienced artist or for free by a new artist working to build up a portfolio. Ask around and see what people are paying and charging. And when you get an artist, make sure you agree on the cost (many artists work by the hour, but they should be able to give you an estimate) before the work is started.
Find an artist. A lot of writers' blogs will list the artist who does their covers, and failing that, it's often listed in the front matter of a book. Look around. I approached my artist initially because I admired a cover she'd done. And it paid off: many people say my novel's cover is one of the most professional indie-pubbed covers they've seen. If you have friends who do artwork, as if they would do a cover for you for a low cost or in exchange for its inclusion in their portfolio.
With a little bit of legwork, you can get a perfectly suited book cover that will work as a great promotional tool.