There's a fun little contest going on over at The Swivet right now.
Query letter, 140 characters or less, punctuation included.
If you go to Publishers Marketplace, take a look at the descriptions on recent book deals. It's a nice exercise to try and compress your entire story into 140 words or so. Though it's not difficult as you might think.
Browse a couple query oriented blogs: Pub Rants, Query Shark, etc. Basically, a good query should convey both the basic plot- and the catalyst that kicks off the story, and what's at stake- why you should care. It should also be well written and engaging, of course. Pinpointing those basic events shouldn't be too hard, you know your story intimately- you should know what motivates your characters, and what they're risking and why it's interesting. Forming that into a cohesive and concise sentence or two is the hard part.
But that sentence is the essence of query zen. I love it.
My entry in the 140 word query contest:
"A dying girl, coerced into acting as grim reaper for a second chance at life, falls in love with the man she was sent to kill."