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I'll give a couple of pointers, then list a few links to other sites with similar articles. It's one of those topics where you need input from more than one person.
First, read your target market's guidelines. If they want three chapters and a synopsis, don't post them the whole manuscript. If they want a query letter and nothing else, do what they ask.
Your query letter does three things: First, it proves you've researched your market. (If your query letter to a romance publisher starts by outlining a thriller set on Mars, you just failed the first test.) The part in which you describe your plot should be sharp and snappy - perhaps three sentences tops.
Second, list your prior publishing credits. If you have something to list, just say "My work has been published in ..." (assuming short story credits) or "My last novel was published by...". Don't freak out if you have no credits - better to skip this paragraph than to print a shopping list of ezines, vanity publishers and primary school writing competitions.
Third, ask whether the recipient is interested in seeing your submission. Be sure to mention the SSAE you enclosed for their response.
One thing about query letters - they don't have to be witty, or amusing, or clever, or the most insightful thing ever to cross the recipient's desk. When you're agonising over the thing, remember that you're just writing a letter to ask whether they'll take a look at your manuscript. In fact, Miss Snark says the following about the ideal query letter:
Just answer these questions:
Who is the protaganist?
What dilemma does he face?
How does it get resolved?
Answer each question in less than 25 words. That's the skeleton for a good query letter. It may not be your finished version, but it will give you the bone structure you need.
If you CAN'T do that...don't query me. Your novel needs the work then, not the query.
There's also a good article on the perfect query letter here, on AgentQuery.com
Now, how would you like a free education in the art of cover letters and sample pages from two genuine agents? With annotated examples? For the first, visit this post on Miss Snark's blog and start reading. (Not recommended for minors as it contains very strong language.)
If you find the harsh realities of the slush pile too intimidating, try Agent Kristin's blog, where she's posted half a dozen query letters which earned the writers representation with her agency. Each has plenty of comments telling you why they were good.
Links to similar articles:
Writing a query letter (includes example): Charlotte Dillons' Page
SoYouWanna write a query letter to a literary agent? So You Wanna