So maybe I will go ahead and talk about blurb etiquette. Remember, this is only a preliminary version. I may think of more helpful hints later, and I welcome suggestions.
When to solicit blurbs.
When your editor or agent tells you to. If you're just hunting for an agent, it's too early. You can't reasonably expect people to do the work of reading and blurbing something that might never get published or might change dramatically between the time they read it and publication. Some agents believe it helps them sell a manuscript if they can present it to the publisher as an attractive package, complete with ready-made blurbs-- so if you're unpublished but agented, discuss the subject with your agent to see what he or she advises. But in most cases, you don't need to worry about blurbs until you have a signed contract and the publishing clock is ticking.
Who to ask
Check first with your editor (or your agent, if he or she's the one asking for blurbs). By all means tell them who you know or who you really admire and would like to read your book. But they're the ones who are working out a plan for marketing and positioning your book. Let their advice guide you. What if, for example, you see your book as a dark, violent neo-noir lightened only by the heroine's sardonic sense of humor. . . and then your publisher tells you that they want you to take out the garroting scene, and they're going to market it as edgy chick-lit. Maybe you need to rethink your vision of having blurbs from Dennis Lehane and Carl Hiassen. (Or maybe this signals the need for a long discussion with your editor about whether they're reading the same manuscript. Whatever.) The point is, blurbs are part of the marketing package; the publisher's marketing department ultimately drives that, and you don't want to work at cross-purposes to them.
Anecdote: when You've Got Murder was still in the prepublication phase, my editor asked me who I knew and could ask for blurbs. I gave them a laundry list--and from my years of being involved in Sisters in Crime and attending mystery conventions, I knew a lot of people. My editor told me which ones to ask. Not necessarily the most famous, or the ones I knew best, but the ones she thought would best give the reader a clue to the kind of book it was. Bless Steve Hamilton, Margaret Maron, and Dan Stashower for taking the time in the middle of their very busy schedules to respond to my request. Bless Jan Burke especially for doing the same thing and not even having the satisfaction of seeing her blurb appear, due to the kind of bureaucratic accident that happens all too often even in the best of publishing companies. (I wonder if they EVER located her email.) And oh, yeah, bless Earlene Fowler equally--she also gave a lovely blurb and I didn't have to ask her; I forget whether my editor or my agent did. That's another benefit of working closely with your editor and agent on this. Sometimes they'll do the asking for you. Which brings me to another topic.
How to ask
Of course, if the person you want to blurb your book is a good friend and has been saying for years, "Damn, when are you going to sell the book so I can blurb it!" you're in luck. You just ask. We should all be so lucky. For the rest of us. . .
I think you can never go wrong with having your editor or agent ask, if they're willing. Maybe that seems a little more formal--like a guy asking his girlfriend's parents for her hand before he formally proposes--but formality has its charms. More important, it puts the whole thing on a businesslike footing from the start. Your potential blurber knows not only who your editor and publisher are, they know that the house really is interested in having them blurb--I've heard of cases where writers solicited blurbs from writers they admired, only to be told that those authors didn't really fit the marketing plan. Or on one occasion, more bluntly, weren't famous enough. Usually (not always) editors are better at giving the potential blurber solid information--when they can send the manuscript or advance review copy, when they need to receive any blurb, how the book is being marketed. And channeling the blurb request through an editor doesn't prevent you from letting your favorite author know how much a blurb from him or her would mean to you; in fact a savvy editor can work that angle quite effectively. They've done it before.
I also think editors and agents are better able to nag blurb targets if it looks as if they've forgotten about a manuscript. After all, we writers are all used to getting nagged by editors about deadlines.
And, frankly, if the potential blurber has to pass, either because of time constraints or because, perhaps, she wasn't absolutely enchanted by the book, I think that's a lot easier for the blurb target to say to an editor, and a lot easier for the book's author to hear from the editor. Then again, I'm constitutionally averse to confrontation.
If you do have to ask directly instead of letting your editor or agent do it, be businesslike. Tell the potential blurber how soon you can send the manuscript or ARC and when you would need any blurb back. Give full information--the name of your editor and your publishing house, and optimally your editor's email and phone number, in case the potential blurber has questions that the editor can most easily answer. And while it's appropriate to mention common ground and shared experiences, don't try to use them to lay a heavy guilt trip on your blurb target. ( "I know I can count on you to help a fellow Sister in Crime achieve her lifelong dream!" is not a good tactic to take. Last time I looked, SinC had 3600 members, and a fair number of them share that dream.)
How to nag
Don't. Remind--maybe once, and closer to the end of the reading period than the beginning. But not the day before the deadline, when the target has no reasonable chance of reading and writing a blurb if he or she intended to and just plain forgot. But if a gentle reminder doesn't work, let go of it. Maybe the person didn't like your book. Not everyone will. Or maybe your blurb target just didn't find time--that happens, too. Just imagine how pleased you will be when the person you wanted to blurb your book comes up to you at a conference and says, "Damn, I wish I'd had time to read your manuscript last year!"
That's it, so far. Offered not because I'm the Miss Manners of publishing etiquette, but because I've been observing and occasionally participating in this blurb thing for a few years and wishing I'd had a set of instructions when I started.