We all do it, right? We scan the shelves at the bookstore, grocery store, or online bookstore and if we don't know the author, we make our decision about a book purchase solely on the cover of the book.
Publishers know how important good cover design is and those with a strong budget and staff, hire freelance cover artists whose works can help them sell books. Some have an entire Design department with a Design Manager overseeing the cover art requests and assigning the work to various cover designers. These folks take the time to not only ensure the cover design is aesthetically pleasing but also meets photo and digital rights requirements as well as marketing scopes which will help sell the book to the right demographic. Not all publishers have the budget or staff allocated to do this but those that do, I've found, tend to sell more books than those who don't.
Regardless of whether your publisher has one of these nifty Design department managers on staff or not, it's important to remember that as the author of the work, you should have approval of the design and a say-so in what will become the visual representation of the work you've spent years perfecting for public consumption. Don't "settle" for a cover you're not happy with simply because you're afraid to "rock the boat" with your publisher.
Make your opinion count and learn to pick your battles. Start with a strong cover art request. It should be distinctive and straight-forward. Cover designers do NOT read your book before designing your cover but it should look like they have. Best way to ensure your cover will reflect the message you're trying to convey is by avoiding "symbolism" or obtuse descriptions in your cover request. Keep it simple! Most authors do well with the character descriptions but when it comes time to explain the context of the book, their flare for the dramatic can be the detriment of their cover design.
Characterization: What do your characters look like? Don't say tan, blue-eyed, blond hero and brunette heroine if your hero is really Hispanic but blond and blue-eyed or you may end up with a very Nordic-looking hero that looks nothing like what you describe in the book. My biggest pet peeve (and my clients will attest to this) is misrepresentation on the cover of any book. It doesn't take much to describe your characters in your cover request. Include body build (gym rat muscle build, swimmer's body, marathon runner, tall and lanky, short and stout), hair color and length (long blond hair versus buzz cut, for example) and nationality if it's prevalent to the story.
Location and theme: Where does your book take place and what is it about? Is it a paranormal thriller set in present-day England or a contemporary romance set in the Caribbean? Knowing the setting and theme of the book will help the cover artist choose the right color palette. If it's a dark, erotic tale, make sure this is clear in your cover request or you could end up with a pastel color palette on your BDSM novel. If it's a contemporary romance set in Arizona, the color palette and setting should reflect the vibrant colors of the Tex-Mex region not the blues and grays of an urban are in Seattle.
Genre: Make sure your cover reflects your genre especially if writing in Sci-fi, high fantasy or paranormal subgenres. If your story takes place in space but your cover has a hilly countryside setting in the background it can be avoided by the readers you're trying to target simply because they don't know it's their type of book. If you're an unknown author or a brand new one, this is could hurt your initial sales.
If you're a self-published author, you know and may appreciate the value of a good cover more than traditionally-published (including dig pub) authors do. When you're the publisher, you have to hire your team and likely design the cover on your own. Unless you're a graphic designer at heart, artist or have some cover design experience, please don't attempt to design the cover on your own. If you have the funds, hire a professional and like any contractor you hire, do your research. Don't base your decision solely on their web portfolio, website or business card design. Send them a cover request for your book and try them out. Choose the graphic designer who conveys your vision on that cover best and for the best price.
As a published author you're part of a team and you need to be a team player, but remember you're the quarterback not the water boy. You call the plays and your team helps you to carry it through to score the winning goal: sales!