From Writers Digest
+Do follow the tried and trusted formula.
Your query letter is not the time to get cute. There’s a very specific standard for query letters that you should not deviate from, when possible. Here it is:
Format with your address at the top of the page, right justified. Next, type the agent’s address, this time left justified.
Use a personalized greeting where you acknowledge the agent by name.
If you don’t have a specific connection with the agent, you should get immediately into the action. The job of this first paragraph is to keep the agent reading.
Keep the body of your query letter from three to five paragraphs.
This is your opportunity to hook the literary agent. Share any connection you have with the agent (you met him or her at a conference, or you’re a fan of specific authors that he or she represents).
In this paragraph, share the title and genre of your book. This is also a great place to include your book’s word count.
That’s right– always include your word count– this is one of the most important information tidbits in your query letter.
Now that you’ve hooked the agent, it’s time to summarize your story. Discuss your main characters, what happens, and what choice they must make now. But don’t give away the entire plot. Leave the agent wanting more by structuring this paragraph in a cliffhanger.
Add your bio, but make sure it’s relevant to writing. Impress your agent with writing awards and credibility (you attend a yearly conference), or related writing experience. Keep it short (no more than two sentences).
Remember with a query letter, you don’t need to reinvent the wheel.
+Do use short paragraphs and short sentences, when possible. Imagine this: your letter is one of hundreds that your potential agent must read through this week. Between you and me, they’re probably skimming quite a few of those letters. Make it easy for them to skim through your letter by employing crisp, easy to digest sentences.
+Do use a similar tone to your narrative. Your query letter should evoke the same tone as your novel. Mirror the same language that you use in your book. Is it funny? Inject some of that humor into your query letter. Is it flowery? Use ornate and elegant language to sell your story. But be careful with wordiness. (See above.)
+Do follow submission guidelines. Be careful about sending off a query letter blitz without reading individual guidelines. Not every agent wants the same things. Some may only accept email submissions while others accept both.
The good thing is, they will always spell it out for you.