I've had this thing for a while now and only used it for class work, but my friend Stewart shamed me into posting, so here goes. As the assistant editor at Dark Wisdom magazine, I see a lot of cover letters. A cover letter is your first impression, yet many that I see make the writer sound desperate and/or don't provide the kind of raw information that tells the reader that this person is a professional I've made a "form" letter that should cover the bases (alter as you'd like and always read a market's guidelines to find out if they have a certain way of doing things) but first, let me tell you some things not to put in one:
Don't put a time limit on how long you're willing to wait before submitting somewhere else. If a writer isn't willing to wait for a reply, he or she is in the wrong business.
Don't challenge the editor. Every so often I get some variation of "This may be too dark for you." Which usually translates as "This is a craptacular slaughterfest." or "The whole plot is about butchering babies." If you think a story may be too dark for a market, it most likely is and you've lost my interest in it as an editor.
Don't give a detailed synopsis of your story. You may want to talk a little bit about the theme of the piece or how you played with common horror tropes, but don't overexplain the story. Why should I read it when I feel like I already have?
Don't ask for a critique. 99% of the time, I just don't have the time to give any personal thoughts on a story. Once in a great while I have a lull where I can help someone who needs it (and usually I get flayed for it on a blog) but it's very rare. I suggest a simple "I'm open to suggestions if you have the time." It lets an editor know you're open-minded but not pushy or needy.
Never give out your Social Security number. I've had this happen a few times now. I already have your name and address, don't send me the final piece in the trifecta of identity fraud. We may need it if we're publishing you and we'll let you know then.
Don't babble. A good many people try to be witty...most aren't. Keep it brief and try not to sound too cool for school.
Anyway, here's that sample cover letter:
Please consider "(Story Name here)" for publication in (Magazine Name here). I'm offering (whatever rights you are providing, usually First American Publishing Rights). Thank you for your time, I look forward to hearing from you.
Note that stories are always in quotes and magazines are in italics. You don't need your address here because it's on the manuscript. When you get published you can list a couple of the mags and books you were in but don't list more than a few because no one cares after they see the biggest names you have under your belt. Again, check your market's guidelines for any requests in this area. Don't kill your chances of getting published over something easily fixed.