Unless you are a graphic designer, hire someone. People do indeed judge a book by its cover. And just as an attractive cover draws the eye, a dull cover can cause readers to move on to a more appealing image. Your best chance of making sure people stop to look at your book cover is to make sure it is designed by a professional.
Judging a book by its cover
The problem that faces self-publishers is how to evaluate an effective cover. "Good" and "bad" reside in the eye of the beholder. Depending on cultural tastes, what is considered "good" can vary widely. Standards also change over time.
Consider Baen Press, a publisher of speculative fiction famous for its ridiculous book covers. (Some of the most horrendous covers can be found on Good Show Sir, along with hilarious comments.) Those covers were not considered awful 30 years ago. (I know because I bought many of them.) Times change, and tastes change along with them.
But are professionally-designed book covers any better? Frankly, I can't tell the difference between what the NYT considers the best book covers, and what Huffington Post says are the worst. Books lists "20 Best Book Covers" that are only slightly less trendy. What is currently popular does not always stand the test of time, or of good taste, so I would caution you against anything that smacks of trendiness.
What are the qualities of a good cover?
The basic components of a good cover are 1) being able to easily read the title and author and all subheadings, 2) an image that doesn't interfere with the written information, 3) a thumbnail that stands out, and 4) the ineffable quality of memorability. Just like a piece of art, a book cover should be memorable.
My idea of a memorable cover is Red Queen by Victoria Aveyard. It is simple, evocative, and aesthetically pleasing.
The theme, expressed beautifully by the image, is spelled out below - for those who need words. And you can easily read both title and author's name. When I close my eyes, I can still see the image and the title. (Those not only stuck with me, they persuaded me to order the book.)
For more memorable book covers - and an analysis of why they do (or don't) work - see Joel Friedlander's Ebook Cover Design Awards.
The bottom line for good book covers is that they make you want to read what's between them.
How do you find a book cover designer?
There are many excellent book cover designers, but locating one who is perfect for your project can be a challenge. To narrow the field, go to Amazon and look at covers for books in your genre. When you come to one that is enticing (would you want to know more about this book based on the cover?) find out who the cover designer is. (You can type "cover design" and the title of the book into a Google search. Alternatively, you can type "cover design" into the "Look Inside" feature in case there is an acknowledgement.) If those methods fail, you can always contact the author (especially if the book is self-published; in traditional publishing authors have no control over cover art). Even if the artists you find through this method do not do freelance work, you now know what you like. When you approach cover designers, you can show them examples of the styles that appeal to you.
Pre-made covers
Pre-made book covers are usually cobbled together using stock images. The way it works is you choose an image you like, the designer adds your name and book title, you buy it, and the image is then taken off the market, never to be used again.
Pre-made covers tend to be quite inexpensive. Cheap Book Cover Depot offers pre-made covers for as little as $5. Fiverr is another service that starts at $5. But while cheap pre-made covers are passable, they have a bland, generic quality that does not make them memorable. If you pay a bit more, you can sometimes find a pre-made cover that looks as if it were commissioned. (Big Sky Words has a list of 10 good pre-made cover designers.)
Individually commissioned covers
Covers that are individually designed cost more (in the hundreds) but will give you the security of knowing your book cover is the equal to anything designed by an artist working with a major publisher. There are two methods of obtaining a commissioned book cover: 1) competitive services in which you place an ad to be viewed by hundreds of designers, and 2) contacting individual designers directly through their websites.
99 Designs is a graphic design service that allows you to post your project in a pricing category ($299-$1199). Depending on your plan, a set number of designers - between 30 and 60 - submit their book cover designs. You have a week to give feedback to designers. After that you choose which design you want. In essence, 99 Designs is a contest.
There are advantages and disadvantages to this system. The advantage is that you get to compare a number of different styles and interpretations, which may broaden your horizons. The disadvantage is that you are under the gun in terms of time. Making a decision quickly, especially if you think the design needs tweaking, may not be in your best interests.
My personal preference is to work directly with a designer. Working directly with a designer allows you to hone your book cover until it is exactly what you want - and the final product will be unique. You will pay more for the design than pre-made covers, but the promise of complete satisfaction may be worth it. I've listed a couple of good designers below. Look at their portfolios and terms to get an idea of what a designer should offer.
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Reputable Designers
Dan Yeager is very professional and reasonably priced. You have to pay half up front, but he won't quit until you are completely satisfied with the final product. Turn-around time is very fast. The cover for my ebook cost less than $250. He also does full print set-ups.
To the left is the book cover Dan Yeager designed for me. It is elegant, memorable, and the text is easy to read.
Alexander von Ness has almost 20 years of professional experience in graphic design and over a decade as Art Director in a branding agency. He is a multiple winner and finalist of international graphic design contests in the category of book cover design. I have not worked with him, but his covers are impressive. Like Dan Yeager, he will do limitless revisions until you are satisfied. Prices are generally in the $400 range.
Von Ness works quickly. He promises a first draft within three days.
Andrew Brown is currently redesigning his website, but you can see examples of his work on his Facebook page.
More:
Mark's List
Smashwords provides a list of affordable ebook cover designers - both pre-made and commissioned work.
While I don't normally recommend making your own cover, if you have an eye for design there are plenty of resources at your disposal.
Making Your Own Book Cover? Best Free Programs for Graphic Design
13 Sites Where You Can Get Fabulous Free Photos
Once you have finished your cover, however, don't immediately slap it on your book. Get a second opinion from a professional. Bioblosson Creative offers cover critiques, as well as cover makeovers at very reasonable rates ($30-$50).