You can't judge a book by its cover. Well, yeah, you can, and millions of readers do it every day online or in the bookstore when deciding what new books to buy. You can write the best new novel of the decade, but if it doesn't look attractive you're never going to get strangers to read your blurb and get sucked in to your story. When you're working with a professional cover designer, you'll be happier with the finished product if you have an idea of a good cover before you start.
Find the Mood
Create a mood board to find the prevailing feeling about your book. Select pictures, samples of fonts, and even short video clips that all evoke the mood you've created in your book. You'll start to see a theme that connects them all. Once your board is full, careful study will show you the direction in which you need to go.
Study your competition. You should never try to turn your cover into an exact copy of one you admire, but successful books in each genre have similar features. Romance novels have people in various states of undress, depending on the story. Thrillers usually include a gun or photo of a government building. Look at the current trends only, as they can change over the years. Yesterday's Harlequin Romance would never fit in with today's 50 Shades crowd.
Don't Dismiss Minimalism
Some of the most striking book covers ever made include a single object and some text on a blank background. If you find one individual item that brings to mind a pivotal scene in your book, use that as an icon of sorts for the entire cover. Even if it only makes sense to those who've read the book, it will catch the eye, and that's what covers are all about.
Never Be Literal
Covers that evoke an entire scene in a book look old-fashioned and dated. You're aiming for your cover to give the reader a sample of the emotion your book creates. Look at the original cover of Stephen King's It. A paper boat, slowly making its way down a puddle toward a sewer, with a few claws sticking through. You can see a whole range of emotions in that cover, and can't wait to read it to find out more. When it comes to covers, give hints instead of handouts.
Don't Be Clever
You may be tempted to ask your designer to use a unique font, one that looks special with your book cover design. There are some very rare examples where this will work, but in most cases you'll end up looking amateurish. Look through a list of fonts and choose a classic serif example. You'll be surprised at the way slightly different fonts can change the look of your cover.
Creating your cover is a creative process that demands your utmost attention. It allows you to see what is working in your genre and to add that special flash of brilliance that will help you break through the noise. If your prospective readers don't see something on the cover that strikes a chord they may never take the next step of peeking at your wrting and really getting hooked. Go for it and make a difference in your work!
Keep the Faith and may the Force be with you!