As the publishing industry has long known, books don't sell themselves. While favorable reviews on Amazon.com and Barnes and Noble are helpful in attracting customers in search of a new read, a lot of groundwork goes into promoting a book's attributes prior to it ever hitting the shelves. The most common of these is the series of enthusiastic quotes (a.k.a. "blurbs") that appear on a book's back cover jacket or within the first two pages. For new writers, these quotes are generally solicited by authors themselves and are from individuals who are recognized as experts on the subject matter or genre. For authors who are already well established, the endorsements are solicited by the publisher.
Study existing book endorsements to get a sense of their brevity (one to three sentences) and structure. These are more frequently found in nonfiction texts and how-to's. The endorsements found in fiction generally come from fellow authors who write the same genre or magazine writers who were privy to galley copies prior to a book's official release.
Ask the author how many words of blurbage she needs and what her deadline is. Query whether there is something in particular she'd like you to focus on in your testimonial.
Open your endorsement with a compelling incentive that will resonate with a prospective reader's needs. This can either be a complete sentence (i.e., "If romance has always baffled you, this book is the key to unlock its mysteries") or a fragment (i.e., "The best of Italy in 200 pages").
Reflect on the elements of the book that struck the most powerful chord as you read it. These elements could be surprising/shocking revelations, rib-tickling humor, eloquence and/or imaginative use of language, pulse-pounding suspense that kept you turning the pages or the way in which a complex subject was presented in user-friendly terms.
Personalize the experience of reading this book. Examples: (1) "In my 30 years as an English teacher, I've never run across a book that made 'Beowulf' such fun to analyze"; (2) "I'm recommending this book to every patient of mine who has been diagnosed with such-and-such"; (3) "If I knew then what I know now about such-and-such, I'd have become a thus-and-so much faster."
Write down all of your reactions to and feelings about the book on a half or full page, then gradually whittle it down to 50 to 75 words (unless the author has encouraged you to write something longer).
Add your name as you'd like to see it appear in print as well as your official title (which, hopefully, has something to do with why you were asked to write an endorsement). If you're an author yourself, it's permissible to include one or two of your published titles--but don't go overboard. The endorsement is to shine a spotlight on the author, not draw excessive attention to your own work.
Send your endorsement to the author for preview. If you feel comfortable with the relationship, it's courteous to invite her to make minor changes to your quotes if she feels she can make them a better fit for her needs. (Most authors aren't going to take you up on this, by the way.)