Do your homework first.
In fact, you can even try it out on someone else first, at Query Eagles. Here's the description of the site by the owner: "I set it up in response to various moanings by agents about the quality, or lack thereof, of query hook paragraphs. Often, as writer, you are so close to your work that you can write what you think is a perfect query paragraph for your novel, but which makes absolutely no sense to a person unfamiliar with the work...The community is about letting a few others read it before you post it off to someone 'important'."
A great idea. And if you post your query, wanting comments, don't forget to comment on other people's queries first as a matter of courtesy.
Other good sites to consult are Pub Rants (a US agent's site). And the now defunct Miss Snark. Do a subject search for their posts on the subject of queries.