I recently attended a novel marketing workshop in Oregon conducted by Dean Wesley Smith, with help from Kristine Kathryn Rusch. While I had some idea about what was needed to sell a novel, honestly I was clueless.
If you ever have the opportunity to attend one of Kris and Dean's workshops - do it. There is no substitute for an experienced professional writer sharing their thoughts, ideas, tricks, and tips with you. That's not even counting the benefit you get from networking with a number of other writers at a similar career level.
You don't have to agree with everything Dean says (though it is generally simpler, since he, like many seasoned professionals, has a low tolerance for argument about things he's seen for himself over the years) but you can gain a ton of insights about the publishing industry today (literally today, as he's constantly tinkering with his workshop content to suit today's specific publishing environment.) And you can identify what in your novel marketing package is working, what needs to be tweaked, and how best to present yourself to a publisher. By the end of the workshop, you will have your novel in the mail to at least two editors.
If you're looking for input on how to land an agent, you may wish to attend a different workshop. This was focused on how to get your novel package dressed up in business dress and in front of actual New York editors, ones who are in the position to buy your book.
A note on the "business dress" of your novel package. Kris, Dean's wife and a highly accomplished writer, says that getting your novel package into the right manuscript format with the right information in it in the right order is like getting dressed up for a job interview. Your packet *is* an interview. You wouldn't show up at a job interview in your pajamas, so your novel package should conform to standard practices so that it rises above the novel packages submitted on crayon or in wacky fonts. You don't want to be the guy in the waiting room in the clown suit, right?
So query letters. There's a general flow to them. Like any business letter, at the top is your name/address/contact information block. Be sure to include a phone number AND an email address. Dean recommends against using the free email accounts like hotmail or gmail, saying they are notoriously difficult to get through to New York Publishers. That's fodder for a whole 'nother post, though, so for now just make sure you have a valid email address up top that you anticipate having active for a minimum of the next 6 months (since your package may not get read immediately.)
Next comes the editor's name, company name, and mailing address. While some publishers accept email submissions, snail mail is still the standard.
BIG IMPORTANT POINT: Make sure that when you list the editor's name up top, you also match your salutation to the editor's NAME AND GENDER. I got the gender right, but accidentally included the salutation to the *previous* editor on one letter. I have to assume that query packet is going into the trash, as that's what I would do if I received a resume/cover letter addressed to the wrong person in the business world.
Let's get into the meat of the query letter. Important point: One page for the letter. One page MAX. And leave enough whitespace so that you can breathe. You can use either indented paragraphs (print standard) or the white line between each new paragraph (the web standard.) Personally I prefer the white line for readability so I include a blank line between each paragraph, that also adds to the overall impression of whitespace. Remember, standard 1 inch margins around, no header/footer, single spaced. You can use a different/smaller font for the letter, but remember it should still be normal-looking and READABLE. You're presenting yourself as a professional, someone an editor will give money to. Don't monkey around with fonts.
What should you talk about in the letter? The thing I think many writers are shy about is talking about THEMSELVES. But this is a little counter-intuitive. When you're selling your novel, you're not just selling these words you wrote on a page, you're selling yourself as a writer of stories, as someone who has things to say, someone to have a relationship with. Ideally your editor will be interested in more than just the one novel you're offering them in this query letter. (However, in general it's not such a good idea to try to pitch multiple-volume novels, particularly if you're a first-time novelist. You can have those other novels/outlines in the wings, but leave them in your back pocket for now.)
Depending on the kind of case you make for yourself and why YOU wrote this book, why YOU'RE the best person to tell this particular story, you might consider leading with a paragraph about yourself. If you've had any contact with this editor before, definitely mention that early as well ("You considered my novel XYZ and suggested I send you other projects...." or "We met at ComicCon at XYZ restaurant with ABC writer.")
After this (or first if you don't have a specific personal opening) - you should talk about your book. This paragraph (just one!) is third-person past-tense, announcer voice. Imagine the voice-over teaser for the next episode of a hot TV show. "When Carrie uncovers a plot to overthrow her pre-school through her teddy bear, she's more than a little surprised, particularly since she didn't realize she could read. But this is the way of it for a pre-school super-heroine, and is just the beginning of Carrie's adventures. Will she be able to save the day, her school, her teddy, and still get home by naptime?"
Think about what your book is ABOUT, not what happens in it. What role does your main character play in the book? One way of thinking about this paragraph is to think about your main Character, in a Setting, with a Problem.
Have I mentioned lately how Dean helped me crystallize this thinking? I knew these things before, but it wasn't until the novel workshop that I really understood them and feel like I can practice them outside the boundaries of this one novel.
If you haven't already promoted yourself in your letter, now is your chance. Who are you? Why are you special? Why are you the best person on earth to tell this particular story? If you have any publication credits, mention them here. Even blogging can be a useful credit if you have a niche blog specialty or garner a lot of readers. But keep it brief. A few lines. What's your personal hook? The thing that makes you interesting? One workshop participant stated he had nothing interesting to say about himself, until he mentioned that he has held sixty-six jobs in his life. That's interesting! Someone like that has stories to tell. I'm intrigued. Be intriguing!
Next up, mention your novel's title, length (there are some exceptions to mentioning length but it usually needs to be up front), and the fact that it's completed. Important! Genre can be mentioned, but in edge cases where your novel might be a suspense novel or a mystery or a thriller, there's no need to mention it. Your short blurb and synopsis (coming later) should tell the editor what they need in order to understand how your book would fit with their line. It could be that they're over-bought on mystery right now, but if they market your book as suspense it would fit well. Don't pigeonhole yourself unnecessarily. 
Lastly, tell the editor what you've included (a sample of your book - typically somewhere in the 15-30 page range, a few chapters, enough to get the flavor of your writing, and a synopsis. Full plot synopsis where you tell the major events of the book including the ending!)
Don't forget to thank the editor for their time/consideration! Depending on the flow, you can also mention that you'd be happy to send the full novel for their consideration (this should go without saying, but it's an important step so the editor understands it is complete.)
Leave a few blank spaces. Then SIGN your letter - in blue ink if possible so it's clear you actually took the time/trouble to print your letter and sign it. And then get it in the mail. Dean recommends priority mail flat rate envelopes. They self-seal (do NOT add tape!) and are becoming the standard way people in the publishing industry interact. It's a little under $5 to mail a packet, which is a reasonable investment. You're a professional, after all. Right?
More in this series soon. But remember - if you have the chance to attend a Kris/Dean workshop - go!
 Note: In my case, I explicitly tell both the age-range and genre of my novel, because the age range is dictated by the age of my character. She is 14, that makes the book a YA book even though it's written with tame themes, her age alone categorizes it as YA. I further specified that it's YA Science Fiction, because there is very little in the market in this space, so I'm drawing attention to it because I hope to find that editor who is just DYING to find some good YA Sci-Fi since there is very little of it. I may be pigeonholing myself a bit, but there's really no other way to dress up this particular pig, seeing as how it's set on a space station and features a high schooler as the main character.