Thank you to everyone who participated in (and is still participating in!) Goblin's Crucible. The following post is my summary of what I think I've learned by doing the Crucible.
As usual, I word this as if I know what I'm talking about, when I probably really don't. However, I do want to share what I've figured out, so please take from this whatever you think is useful, and otherwise, just trust your instincts! You're the writer, and you know what you're doing. :-)
How to write a query letter pitch:
If you can write a great book, then you can write a great query letter, because an effective pitch requires nothing more than the same elements a gripping novel requires.
You have stringent space constraints in a query letter however (one paragraph, possibly broken up into two or three short paragraphs for dramatic purposes), and for this reason, you want to slurp the reader's interest into the story as efficiently as possible.
So let's think about how you first grab the reader's interest in a novel:
Your story's inciting incident--the event that kickstarts the whole plot--is the one thing that convinces a reader to plow through another three hundred pages to find out what happens. For this reason, the inciting incident is also the thing you want to focus on in your pitch. It's the event that most efficiently makes the reader want to read more.
The inciting incident will provide the body of your query pitch paragraph, but it isn't quite enough by itself. The purpose of a pitch is to convince an agent or editor they want to read the whole book, so you want to give them the sense that there is a much larger story about to unfold. You also want to make them intensely interested in finding out how it unfolds.
Since you can't make a person curious by satisfying their curiosity, you do not want to explain the book's plot in a synopsis-like manner. Instead, you want to get the reader emotionally attached to finding out what happens. So--let's think about how that gets accomplished in a book:
Emotional attachment occurs when you introduce the reader to a sympathetic character, and then put that character into a situation that demands the character must act in order to avoid some horrible outcome. The reader is drawn deeper into the story's plot by (1) increasing stakes, (2) intensifying conflicts, and (3) deepening mysteries. (There are probably other things, too; add to this list as you see fit.)
So these are the things you want to include in your pitch also.
Creating a sympathetic character means adding something to the pitch the reader can emotionally relate to. A bit of tragic backstory might make them feel empathy for the character, and showing a laudable personality trait makes the reader want to cheer that character on. You must be very sparing with this stuff, however, because backstory and superfluous elements kill your pitch simply by slowing it down too much.
Your rule for including extra information should be: If it doesn't increase the impact or drama of the inciting incident, remove it. You haven't got space to include pace-killing filler; you must focus on those things that whet the reader's appetite for reading the book--which are the elements detailed in the next paragraph.
(1) Increasing stakes, (2) intensifying conflicts, and (3) deepening mysteries (ect.) are the plot "intensifiers" that draw the reader deeper into the story. Since these suck the reader into the book, you really want to pack them into your pitch also. Due to space constraints, however, you probably only want to focus on two or three of them.
Inspect your story, especially right in the wake of the inciting incident, and pick the plot intensifiers that are most dire, and which will most obviously force your protagonist to take action in order to avoid disaster. Again, remember that your purpose is to make the reader curious about how the situation will be resolved, so focus on the buildup and don't talk about the resolution. At the end of the pitch, you want the reader to understand the protagonist is in a horrible jam, but to only have the vaguest idea how the protagonist might get his-or-herself out of that jam.
The last sentence of your pitch in particular should contain some kind of teaser that makes the reader very agitated to know more. Again, think about the plot intensifiers that follow directly from the inciting incident. Now, pick the one that increases the stakes the most, or ratchets the interpersonal tensions up the most, or which is the most fascinatingly mysterious. This is the thing you end the pitch with--you leave the reader dangling, wondering about how that one last oh-so-horrible complication can possibly be resolved.
In summary, your pitch should be structured as:
1) Inciting incident (with a very sleek minimum of world-building, empathy-building and backstory attached to it)
2) One or two complications that make the protagonist's situation more dire and which will obviously demand action from the protagonist
3) One final whopper of a complication that leaves the reader intensely curious about how that complication can be resolved
The good news is, all of these things are already in your book. Your task is (simply; ha!) to figure out what they are, and use them to write a pitch that is juicy, riveting and which makes the reader so rabidly curious about how the story resolves that they must read the book. Good luck!
A useful trick for verbal pitches:
Spoken dialogue in screenplays is often structured with a modifying phrase at the beginning of the sentence, such as:
If we're going to save the farm, we have to sell that horse.
The reason why this is done is that it forces the audience to pay attention right to the end of the sentence. The modifying phrase lets the audience know that they've only heard half the thought; they need to keep listening if they want to know the full story. It's easier for a person to let their attention drift when the sentence is structured as: We have to sell that horse if we're going to save the farm.
If you're giving a verbal pitch to an agent or editor, you can use this trick to hang onto their attention on a sentence-by-sentence basis. Structure what you say so your sentences have a modifying phrases at their beginnings. Read the two examples below aloud and see if you agree that your attention would wander more during one of them than the other:
1) Jimmy will be stuck taking his sister Lila to the dance unless he can convince the lovely Stella she'll look better on Jimmy's arm than on the rakish Cole Stanton's. Lila is the one who can teach Jimmy to waltz beautifully, however, and she won't teach Jimmy a thing unless he does promise to take her out. Jimmy can get his lessons and get out of his obligation if he can convince his friend Rhett to ask Lila out. Rhett thinks he might be able to woo Stella away from Cole also, unfortunately, and the last thing he wants is competition from Jimmy.
2) Unless Jimmy can convince the lovely Stella she'll look better on his arm than on the rakish Cole Stanton's, Jimmy will be stuck taking his sister Lila to the dance. However, Lila is the one who can teach Jimmy to waltz beautifully, and unless he does promise to take her out, Lila won't teach Jimmy a thing. If he can convince his friend Rhett to ask Lila out, Jimmy can get his lessons and get out of his obligation. Unfortunately, Rhett thinks he might be able to woo Stella away from Cole also, and the last thing he wants is competition from Jimmy.