Often, especially for a new author with a new book, it's just not possible to get the attention of a a superstar or an expert who will add authority to yours.
That doesn't mean your book has to be blurbless.
There's nothing wrong with asking for and printing blurbs from friends and family, if it's appropriate to your book. Later on, if Martin Scorsese or another celeb falls in love with your words, you can revise the cover to incorporate the new comments.
My first self-published book I Only Flunk My Brightest Students: stories from school and real life, deals with my life. So it made perfect sense to use blurbs from people who know me, rather than some distant Nobel Prize winner.
The book is funny. Identifying Howard Krosnick, the source of my front cover blurb as "author's classmate since first grade" is almost a parody of the traditional stuffy IDs ("professor of Indo-Eurasion folk medicine at the University of Guatemala), and reinforces the mood of the book.
Stories I'd Tell My Children (but maybe not until they're adults) is an updated replacement for the flunk book. It has a fantastic cover blurb which says, "This book is so funny that I nearly peed in my pants. My girlfriend didn't think it was funny, so I got a new girlfriend."
The blurber, Nicholas Santiago, is someone I know through business. His words are sufficient. I see no need to explain who he is, and I doubt that Oprah could have written a better recommendation. I received "five stars" and some nice words from the Midwest Book Review -- but those words are not as funny as Nick's words.
There's nothing wrong with your acting as a writing coach for your blurbers. You can even write a complete blurb and ask someone to "adopt" it.
If you’ve written a how-to book, the best blurbs will come from people who have actually been helped by it.
A good way to find “amateur” blurbers who might write sincere comments about actually benefiting from your book is to observe online communities that are concerned with your subject. If you find articulate people with problems your book solves, offer to send them free advance copies (even PDFs if bound copies or ebooks are not yet available) in exchange for their comments. You can say that you’d like to know if the book was helpful and how it can be improved. Mention that you might like to quote their comments, but don’t guarantee it.
Here's a great blurb, from a new author, for one of my books about publishing: "Michael Marcus’s book on self-publishing was detailed, complete and easy to read. It is the best I have read on the subject. It was very helpful. I do highly recommend this instructive book to anyone who wants the complete instruction guide to getting your written works out there.—Charles Eastland, author of The Fire Poems"
- If you get a good blurb, identify the blurber in some way that may help her or him. In an ebook or online, provide a link. Like it or not, blurbing is often mutual ass-kissing. Play the game if you want the benefits.
- If you have a connection to a real celeb it may be tempting to ask for a favor -- but make sure the fame is relevant to your book. If your college roommate lives next door to super-chef Mario Batali, Batali's comments about your book about bicycle repair probably won't mean much.
- Beware of self-serving blurbs that say more about the blurber than the book, or blurbs that were obviously written without reading the book.
Short blurbs are usually better than long blurbs. Humorous blurbs (if appropriate) are often better than serious blurbs.
Request blurbs as long in advance as possible -- as soon as you have a draft of your book that is good enough to show. The book does not have to be complete. You can probably get by with an introduction, a table of contents, and a few chapters sent as a PDF. If you want a blurb from someone famous, it’s probably better to send an ARC (Advance Review Copy) than a PDF.