Rather than run the risk of repeating Anton and Stacia, I'm going to take a different tangent. Since the only successful query letter I've written was basically a threat (I've got an offer coming in are you in? I could go somewhere else. You're in?), I'm no expert. Over time though, this stuff just sinks in.
Let's take some time to prepare for writing your query letter. Assuming you've already edited the shit out of your manuscript and the writing's tight as the ketchup cap in my fridge (permanently attached to the bottle at this point), you should probably get your work out to some agents. What do you need to know?
1. Know Your Genre
You'd be surprised how many people I've met (or maybe you wouldn't), who, when asked what genre they write, say, "It's sort of a hybrid of sci-fi/fantasy and mystery, with comedy elements. Oh and it's inspirational like The Celestine Prophecy. I think it'd probably be shelved in mainstream fiction." Yeah. No. No. That book isn't getting shelved anywhere, except that round recepticle with the mold spots in the bottom. I can't stress enough the need to figure out where your book fits into this whole publishing world. If you don't know, how can anyone else?
Exercise: Field trip to the bookstore. Take a look at books that are similar to yours. Where are they shelved?
2. Know Your Target
While you're at the bookstore, you're going to find some agents to send that query letter out to. Check the acknowledgment sections of those same books that are like yours. Authors tend to thank their agents. It's something about getting us our money. I think. No ones really sure. We just do. So take advantage, write those names down. Look them up later on the internet, or in the Writer's Market (but don't buy it or your stock will plummet). You'll want to mention that you like their client so-and-so's book in your query, so this bit's important. Check out Publisher's Marketplace, there's a subscription fee, but it'll show you who's selling what and in some cases for how much (roughly), then you can cancel after a month of researching. Still cheaper than buying the Writer's Market.
3. Know Your Story Structure
Before you actually write that letter, you better know how to describe story structure, even if you don't actually describe it in the query. The biggest problem with hooks (and manuscripts for that matter) is a clear lack of understanding of classic story structure. What sets the story in motion? What is the conflict? Is there escalation? Climax? Resolution? Is there an apotheosis? Joseph Campbell is a great source for story structure, so are certain Russian gentlemen.
4. Get Crass and Sell
This last one's self explanatory. Sure you're a creative artsy f*cker. We all are. But, when it's time to sell, it's time to sell, and publishing is a business, first and foremost. Look at your work from a marketing standpoint. What are the selling points. You may not write these in the query, but you should know them. They'll help you define your hook (what are the best parts, what aren't).
Now you're ready to write your query letter. Simple as that. Be sure to breathe. That's also important.
*This post brought to you by mental exhaustion. I'll not be held responsible for bad grammar after midnight.