While we’re on the subject of queries, here a few tips based on what I gleaned over the various writing and agent blogs and resources.
250 words or less for the entire query
Agents have differing opinions on personalization, so go with what that agent requests or if they don’t say, go with your gut
Don’t include too many characters
Say what the main character wants, what the obstacles are, and what the stakes are if she fails
Match the tone of your story unless it would be gimicky
Don’t be gimicky
Don’t use first person
Use only one POV (didn’t find this out until draft 49. :-\ ). Mention others, but don’t synopsize them
Don’t sound too much like a movie trailer
Don’t reveal the ending (you want to entice the agent to read on)
Be professional but let enough of your personality through that you don’t sound like a robot or an answering machine (that’s the part I have trouble with)
Best sum up of advice: you are synopsizing the first 20-50 pages only
Research the agent, address the letter specifically to that agent, and follow the agent’s guidelines.
Include genre and word count (rounded to the nearest 1,000—I finally found that somewhere).
Include comp titles so the agent has an idea what he or she is reading. I had trouble with this until I figured out you don’t have to include something similar to your work. You can, but if, like me, you can’t find anything, include what you think your readers would also like. Sadly, these comp titles are supposed to be recent. Hopefully three years but five years at most. That ruled out Lord of the Rings and Sabriel for me. I finally went with fans of The Avengers movies. That was something else I learned. Comp titles can be TV shows or movies.
A lot of this is probably subjective by agent. Thankfully, agents know that queries are harder to write than books and that we often are terrible at it. So do your best, but if agonizing over it keeps you from sending, just send it! (that was the best advice I found)