What I wish someone had told me when I first started submitting short stories for publication.
Edit: The link seems to be broken now; in trying to track down its new location, I found this.
Bottom line, though: cover letters are business communication, not a college application. They should be short and to the point, and should signal that you’ve read the guidelines of the market you’re submitting to. The message you should be sending is “I will be calm and professional about this potential business transaction” not “I am the greatest author, this is the greatest story, and you should rush to publish it.” The story itself may or may not deliver the latter message; you’ll just have to trust it. The former message is much harder to get across, but is much more useful to an editor who’s on the fence about buying a story.