As you may have surmised by this point, I am not an agent! I have never been an agent, I'm not sure I'd ever want to be (or am cut out to be) an agent, and so there's no reason to think I ever will be an agent. Aside from having written a few dozen query letters in my day and reading many an agent blog, I have no direct experience with actual, paper-and-ink (or electron-and...more-electron) queries.
But! I have written pitch letters, and I do work in sales, so (to some extent) I'm very familiar with many of the basic components of query-writing and -reading, so I consider myself qualified to at least talk about the basics, which (as you may also have surmised) I now will.
Less is more. I'm led to believe that agents don't have a ton of free time. Your query, like a pitch letter or title presentation in a sales call, has to be short and sweet. Yes, there's more small talk and relationship-building between a sales rep and a buyer than between a potential client and an agent, but a good salesperson knows when to be social and when to be businesslike. I'm not saying not to have a little fun with your query; what I'm saying is, cut to the chase. Keep it under a page.
Be professional. This sort of ties into the above point, and it also kind of goes without saying, but it bears repeating. Besides being as brief as possible, you want to be polite and professional. Do not call your novel a "fiction novel," do not talk about how it's sure to be an instant bestseller, do not talk about your multiple academic degrees or your sunny disposition or your cat. Talk about your book, and if it's a non-fiction proposal, talk about yourself insofar as it pertains to the project you're pitching. That's it!
Personalize, personalize, personalize. Guess how many non-personalized pitch letters to editors, publicists, and other industry professionals go into the so-called circular file? Around 95 to 100 percent. It's the same deal with agents: don't be creepy and tell them how much you like the floral wallpaper in their living room and by the way could they please turn the TV toward the window so you can watch reruns of Get Smart with them, but at least do them the courtesy of addressing them by name (no "Dear Sir or Madam"s or "To Whom it May Concern"s) and demonstrating that you know something about them and their agency. Mention some titles they've represented that you liked! Tell them you thought their post on query letters was really helpful! Don't get carried away, but if you expect an agent to take the time to read your query (and hopefully, your partial and full), take the time to personalize your query.
Follow directions. Yes, it can be frustrating when one agent asks for a 300-word double-spaced query and another asks for a 500-word single-spaced query. Occasionally you will find that different agents want totally different—perhaps contradictory—things. But if you believe that agent is right for you, take the time to tailor your letter to their guidelines, which (one must assume) they have established for a reason. If they ask you to include the first ten pages, include the first ten pages, and don't send a writing sample (no matter how sorely tempted you may be to do so) if they specifically ask that you don't. You want to put your best foot forward from the get-go, and following an agent's guidelines is a very big and generally necessary component of achieving that.
Do your research. This ties into the above point, but in a more general sense. If an agent doesn't usually represent science fiction, your grand space opera spanning 10,000 years and a half-dozen galaxies probably won't interest him or her, and you'll likely waste both your and the agent's time by querying. If it's not clear from an agent's guidelines or title list whether they represent your genre, by all means, go ahead and query anyway; however, 90% of the time, you should be able to figure out whether an agent will be interested in your type of project based on his/her (agency's) website. You're not looking for just any agent, after all—you're looking for a business partner, one who's genuinely interested in your work and willing to champion it to an editor. In short, you're looking for a good match.
Know how to sell your product. Sure, you know your product; after all, you wrote your book, so you know it better than anyone. Your knowledge of your book isn't being tested, though, but rather, your knowledge of how to present it. If I'm writing a pitch letter, it's not enough that I know everything about the title I'm trying to push—I have to know the best way to position it and anticipate what will catch the reader's eye and hold his or her attention. You need to know that about your product—your book—as well. Where's your hook? What sets your paranormal romance apart from all the other paranormal romances currently on the market? Don't start crunching BookScan numbers or hypothesizing about your target audience, but grab and hold the agent's attention with a great opening line and a well-paced, concise description that leaves him or her wanting to know more by the letter's end.
That's all I've got for you, gentle readers, and I hope it's not a total rehash of all the query advice you've gotten before. As always, if you have any questions or comments—or even rebuttals, calls of shenanigans, or plain old-fashioned vitriol—fire away!