If you think people don't judge a book based on its cover, you've probably never read one. Not only do people judge books based on the cover ... but they do it within the first eight seconds. So, the absolute best way to KILL your book's chance of ever being read is to give it a really, really shitty book cover design, so everyone who walks by it will instantly decide your writing is of the same quality and move on to any of the other ten thousands books to the left, or to the right of it.
One of the best ways to ensure your book cover design is as absolutely shitty as possible ... is to design it yourself. Unless, of course, you're already a designer with experience who happens to have written a book, and even then, you still may not be the best choice to create the cover for your book, as your perspective on the material is completely different from where (potential) readers are sitting. The author knows the material inside-out, a reader must be able to see enough on the outside to give them a strong enough reason to look within it.
Since 2000, I've designed and published over 80 titles. (http://www.innercirclepublishing.com/) If you looked at the first one I created in 2002, and compared it with our latest release, you'd see of world of difference in both the quality of design and original graphics. It seems the more we do something, the more we learn about it. Which brings us to the next method by which an author can effectively KILL their book: To design and create the book cover themselves, having never designed one before.
Just because you write, does not make you a designer. Just because you have ideas, does not make them 'good' designs. Over 90% of our covers I've created using the following method: Find out what the author has in mind, then use my experience in design and graphic arts to create the best possible cover based on the author's vision and my collected experience. This results in a true compilation of the author's original ideas combined with the quality of a designer's experience.
So here's the absolute BEST WAY FOR AN AUTHOR TO KILL THEIR BOOK: Have really, really, really shitty ideas and absolutely no eye for design, whatsoever, combined with absolutely no experience at all ... THEN (and here's the key to really failing at this) ABSOLUTELY REFUSE to listen to, or accept, even a single idea from those who are actually book cover designers, you know, those people who do this for a living.
To really KILL a book, an author needs to come up with an amateur design, something so stupid and ugly and unfitting to the rest of the book that no one will ever be drawn to read it, ever ... then STICK to that stupid design no matter how many good-intending, experienced professionals tell you otherwise. After all, they are trying to help the book become successful and if you listen to them, it could be. After all, it's not about what's best for the book, what's really important is making sure that design you drew on your 10th grade notebook gets used as the cover for your first book.