Hire an artist or have some artistic input. If you are the author of the book or not an artist yourself and your vision of the cover is not within the means of your talent, it might be best to get a professional artist to give it a try.
Find out what the book is about. We all know the saying you can't judge a book by its cover, but often times that is the deciding point when people look for something to read. Be sure to either illustrate, or paraphrase what the book is about if possible.
Brainstorm ideas and designs. Mixing and matching ideas for the cover is important to get the right feel. You may also want to keep in mind your choices of having a hardback, paperback, or spiral bound book.
Choose the right font. Having the right font can change the style of the cover drastically. This is another area where having multiple choices and maybe mixing some can make a big difference.
Get a second and third opinion. Although you might have the final say; it is always good to have more than one opinion. Since this book may also be seen in public, perceptional opinion requests are valuable and take minimal time.
Make multiple samples of ideas before choosing the final book cover. There is nothing like being able to see all of your ideas and choices in their entirety right there in front of you. This will allow you to see the finished products side by side, possibly making the elimination an easier choice.