The Great Agent Hunt is like any other hunt. You must stalk your prey. You must find it's territory, lay your traps, and bait them with what your prey likes. This is the article where we talk about Agent bait, AKA The Query Letter.
Before we get into the tips, remember this: anything and everything in this list can be tossed aside if the blog/website of the agent says they want something different. If the Agent wants your query letter in crayon on lavender paper scented with Chanel No. 5, it's off to the perfume counter with you! But if they don't specify, here are some quick rules of thumb.
Unless specified otherwise, a query letter is an actual, on paper, physical letter.
Get the salutation correct. Make sure the name is spelled right and you have assigned the correct gender marker to the agent.
Learn what a real business letter looks like. Business email format, unless specifically requested tends to annoy Agents and their readers. What you want is a real, old fashioned, indented paragraphs, properly formatted business letter with all the commas in the right place and the closing center justified.
Spell check, grammar check, do it again, and then have someone else do it for you as well.
Finally, make sure your contact information is in there and correct.
None of these seem like a big deal, but get any of them wrong and you can get booted before the reader has even gotten to your plot synopsis.
Less Quick Tips:
Be professional and polite. You are trying to sell your work to this person. Pretend you were going to buy a car. How would the ultimate, best-ever car salesman approach you? What would he do? My guess is he's treating you with a lot of respect, and not trying to make you think the Subaru you're looking at is a Rolls Royce. Unless you're rather atypical, he's not hard selling you. He's just gently, respectfully convincing you that he's got the car of your dreams.
The heart of sales is this: Your job is to make the person you're selling to understand that he needs whatever it is you've got. That means showing the Agent that your book is a great match to his area of specialty. It means showing the Agent that you know what that specialty is, and why your book compliments it. For example: if you were querying JABberwocky Literary Agency (Charlaine Harris' Agency) you would explain why your books are similar to (but not copies of) the Sookie Stackhouse books and how they would compliment the portfolio of books represented by JABberwocky. Side note: If you have written a book about a bunch of quirky vampires in the South, it might be a good plan to not query JABberwocky, they've already got the bit of their portfolio filled.
Not Quick At All Tips:
Develop a sharp and witty why you picked this Agent opening paragraph. Do it without sounding like sycophant or a stalker.
Develop a one paragraph description of your book that is meaningful, engaging, sharply written, and makes the Agent want to see more. Description does not mean back of the book copy, and it doesn't necessarily mean synopsis. You need to get the idea of the book across, but anything from dwelling on the themes of the book to the plot to who the characters are may suffice. Pick whichever aspect of your story best matches the work of the Agent and play that up.
Explain who will buy your book, and no matter what, even if the book is Harry Potter and the Secret Promise of the DaVinici Code the correct answer is not "Everyone!" Take the time to actually research your audience. Now, here's the balancing act, this is not a marketing report. The Agent is not looking for something along the lines of: "According to our polling data this story line is especially popular with Caucasian females ranging in age from fourteen to twenty-one in upper middle class homes in the suburbs of moderate sized cities." You are looking for more along the lines of what Amazon does when it tells you that people who liked this, also bought that. So, for example, if everyone who liked your book liked Twilight, you might mention that your book should appeal to Twilight readers. If your readers liked something in specific about Twilight that you can home in on all the better. For example, the readers who were quite entranced by the Werewolf/Vampire politics of Twilight adored the supernatural realpolitik that showed up in your book. Show the Agent that you have a market, and who that market is.
Finally, tell the Agent why you are the person to tell this story. If you've already got writer cred here is the place to put it. If you don't then talk (briefly) about why you can tell this story well.
Wrap up with a polite "Sincerely" and then send it off.
Don't forget the self addressed stamped envelope! You want a response, right?
(Oh, and lastly, it should be one page long.)
Next up: What the letter physically looks like and writing the 'What Your Book Is About Paragraph of Doom!'