That's what I'm doing today, and I've come up with some guidelines for myself--and resources others might like.
How to Write a Query
First word should be name of main character.
First couple of paragraphs stick with the story.
Who wants what, why, and what's in the way?
Emphasize the conflict. Don't just let it be characters wandering around.
Don't open with a generality or homily.
Do NOT start with a rhetorical question. The answer the agent comes up with will not be what you want.
AVOID the passive like the plague. Have it be actions by protagonists.
Don't say "so begins" "the theme is" "timeless question" or "about."
The weather is not relevant.
Remove non-key info
Be specific. Avoid generalities and cliches and summarizing. Include telling details
Avoid too many characters and names.
Include whatever details make characters sympathetic--this is not just about plot, the character needs to speak through the query. But not literally.
the sentence that every single query letter needs: the point at which the action starts. Where the protagonist must choose what to do. Should be in the 1st/2nd paragraph.
Closing sentence, title of work, genre, wordcount--NOT themes or "what it focuses on"--the pitch should have shown that.
Don't say this is your first novel. Of course it is, unless you've actually *published* another novel.
Don't mention trilogies or whatever else you've written that you could sell. Stick to one book.
Just end with "Thank you for your consideration." They know you want them to read more/represent them/etc. (Unless you can give specifics as to why you chose them.)
Try to keep query under 250 words.
For email queries, put your contact info at the bottom, not at the top (like a letter).
Nathan Bransford's query forum: http://forums.nathanbransford.com/viewforum.php?f=12
QueryShark's query critiques: http://queryshark.blogspot.com/
Jodi Meadows' query project: http://jmeadows.livejournal.com/779229.html#cutid1