What's It About? - How to write the stuff on the back of the book

Out of all the books, your cover caught a reader's eye. Now, quick - in less than 100 words, sell your story to the reader. What's it about? Why's it interesting enough to download a sample - or pay for, sight unseen?

I know, I know. If you could say everything interesting about your novel, it'd be as long as... your novel. Welcome to the wonderful world of ad copy, also called promotional copy, and generally referred to as "the blurb." (Which is a misused term, as "blurb" used to mean solely the line on the front cover by famous author saying "greatest thing since hot running water!" But misused or not, it's becoming the standard term. I still think "blog" is a silly term, but just stand aside while the relentless changes to the english language roll on...)
First, just as the book cover is an advertisement not a true representation of your book, the blurb is an advertisement, not a plot summary. If you are describing what's going on past chapter 3, you're doing it wrong. (Unless it's epic fantasy.)
Second, all blurbs, regardless of genre, boil down to three critical elements. A protagonist, in a place, with a problem.

If you have 50 minutes to spare (actually less than that, once you skip the intro on each video), watch Dan Wells talk about story structure here: http://www.youtube.com/watch?v=KcmiqQ9NpPE&feature=share&list=PLC430F6A783A88697

If you don't, answer these questions.
Who is your major character?
What is your setting?
What is your major conflict?
What is your story hook - what is your starting state for the book? (this is usually the opposite of where the protagonist is at the end of the novel / character growth arc / action arc.)
What is your plot turn - what introduces the conflict, and sets the plot in motion? (confronting new ideas, getting a call to adventure, etc.)
What is your pinch? (The point where everything goes wrong/ your characters have to take action.)

There's a whole lot more on story structure, but this is all we need for the blurb. No, really.

"Percy Jackson is a good kid, but he can't seem to focus on his schoolwork or control his temper. And lately, being away at boarding school is only getting worse-Percy could have sworn his pre-algebra teacher turned into a monster and tried to kill him. When Percy's mom finds out, she knows it's time that he knew the truth about where he came from, and that he go to the one place he'll be safe. She sends Percy to Camp Half Blood, a summer camp for demigods (on Long Island), where he learns that the father he never knew is Poseidon, God of the Sea. Soon a mystery unfolds and together with his friends -- one a satyr and the other the demigod daughter of Athena -- Percy sets out on a quest across the United States to reach the gates of the Underworld (located in a recording studio in Hollywood) and prevent a catastrophic war between the gods."

Your Protagonist is at his Hook / Starting State. When Plot Turn conflict happens, he is sent to second location/introduction of supporting main characters, where he learns Information that sets Plot In Motion. Then Percy Starts Action Plot Arc because Major Conflict of Story.

"The Son Also Rises . . .

On a near future Earth, Good Man does not mean good at all. Instead, the term signifies a member of the ruling class, and what it takes to become a Good Man and to hold onto power is downright evil. Now a conspiracy hundreds of years in the making is about to be brought to light when the imprisoned son of the Good Man of Olympic Seacity escapes from his solitary confinement cell and returns to find his father assassinated.

But when Luce Keeva attempts to take hold of the reins of power, he finds that not all is as it seems, that a plot for his own imminent murder is afoot—and that a worldwide conflagration looms. It is a war of revolution, and a shadowy group known as the Sons of Liberty may prove to be Luce's only ally in a fight to throw off an evil from the past that has enslaved humanity for generations.

Sequel to Sarah A. Hoyt's award-winning Darkship Thieves, and Darkship Renegades, this is Book One in the Earth's Revolution saga."

Tag Line.
Setting. Protagonist in Starting State, with Plot Turn conflict.
When Protagonist moves to secondary setting, he learns Information that sets the Plot In Motion. Introduction of allies and of Major Conflict of story.
Information on Series.

Note - Percy Jackson has a lot less initial setting, because it's set in the "Real World, modern day." The further from baseline normal you get, the more explaining you have to do.

"Revolution rises!

The Interstellar Empire of Man was built on the enslavement of the gentle Stardogs, companions and Theta-space transporters of the vanished Denaari Dominion. But the Stardogs that humans found can't go home to breed, and are slowly dying out.

As the ruthless Empire collapses from its rotten core outward, an Imperial barge is trapped on top of a dying Stardog when an attempted hijacking and assassination go horribly wrong. Trying to save its human cargo, the Stardog flees to the last place anyone expected - the long-lost Denaari motherworld.

Crawling from the crash are the Leaguesmen who control the Stardogs' pilots by fear and force, and plan to assassinate Princess Shari, the criminal Yak gang, who want to kill everyone and take control of a rare Stardog for their own, and an entourage riddled with plots, poisons, and treason. But Shari and her assassin-bodyguard have plans of their own...

Stranded on the Denaari Motherworld, the castaway survivors will have to cooperate to survive. Some will have to die.

And some, if they make it to the Stardogs breeding ground, will have to learn what it means to love. "

Tag Line.
Setting! Setting includes aliens and aliens' conflict!
Protagonists (the whole group) in Starting State, with Plot Turn conflict.
When Protagonists move to secondary setting, introduction of protagonists and Information that sets the Plots (All of them but two?) In Motion.
Introduction of Major Conflict of story.

Now, some more important notes.

Note 1: After you write the intial draft, go back and highlight every past tense and passive voice usage. Eliminate them all. Past tense is for reporting what happened. Present and future tense are for getting people interested at what is happening, and what will happen.

Note 2: So, your protagonist has a bunch of friends who helped him, and a mentor, and a mother who saw him off, and people and people.... Almost none of these need to be mentioned in the blurb. If he doesn't find out who is holding his kids hostage until chapter 22, it doesn't go on the blurb.

Note 3: Use adjectives and adverbs. This is like poetry - every word needs to be doing something, or it doesn't need to be there. Haiku gives you about 17 syllables in English; blurbs give you about 100 words. So use connotations, denotations, alliteration and implication!

There is a difference between that and hackneyed cliche. Don't go cliche, man. Readers' eyes glaze over so fast they look like Krispy Creme donuts when they hit cliche, because they assume the story will also be full of hackneyed cliche... Just Don't Go There.

Note 4: When you think it's pretty good, try to tell a friend about this really cool book you found... using your blurb as memorized, not as written and consulted. You will find, when text is converted to speech, that not only are a lot of hand gestures involved, but that you'll start dropping words, phrases, entire sentences that sounded really cool when you were writing and editing, but aren't going to keep your friend's attention. Go back and edit with this in mind.

Note 5: There is a form of pitch known as "the elevator speech." It often is presented like this: you get on an elevator, and realize the other guy on it is an acquiring editor / your favorite director / an actor you want to get really excited about your story/screenplay/novel. To break the awkward silence of the elevator, they peer at your con badge and say "A writer, huh? What's your book about?" You have three to five floors to sell your story, off the cuff and impromptu (but you do have a short speech prepared for this moment, don't you?). Go.

Yeah, that's your blurb. Instead of Adam Baldwin or Toni Weisskopf, though, you're trying to reach through a computer screen and get your money from a bored browser who wants something good to read, and has been momentarily distracted by your book cover. You want his money? You have three to five paragraphs. Go.