Cover Letters

Cover letters are often the first thing editors and slush readers see, and it's where you have the opportunity to shine. Done badly, though, they can turn off the reader. Here are the basics every writer should know:

Cover letters only really matter when publishers request them in their submission guidelines or a space for one appears in an online submission form. Several places don't really want them unless they're considering your work, but those that accept cover letters offer an opportunity to say something that elevates your story or novel above the rest of the slush pile. You want to come across as professional, even if you're not pro-published, and you do that by establishing your credentials:

Have you published stuff? List three places your work has appeared (it's okay if they're not paying markets; any publication shows that editors like your stuff enough to go through the publishing process with you).
Haven't published before? For short stories, that's cool, because short fiction magazines don't operate so much on Big Names as they do on Great Stories. Every editor wants to be the one to publish you first. For novels, it's more complicated, but that's why you have an agent.
Have you done stuff that demonstrates you're working toward a writing career? List things like professional workshops you've taken (mine and Kij & Barbara's count!) and other relevant studies (seminars, MFA, etc). This goes well with the not-published item above. If the work you're submitting went through workshop, that's a good way to say you've both gone to the effort of taking a workshop plus the item at hand isn't in rough-draft condition.
Are you a subject-matter expert? Especially in spec-fic, this can be a powerful selling point. List technical, scientific, or other workshops, seminars, degrees, work experience, and so forth that are relevant to the story.
Expertise that extends beyond the academic: Did you live where the story is set? Have you played in a band as the protagonist does? Have you survived drowning like they did? And so forth. This shows you're writing from experience and the work is more likely to feel real and immersive.

Otherwise, just follow the publisher's guidelines. Keep it short and straightforward, because the cover letter isn't the work you're submitting. And save the text you use for a submission so you aren't starting from a blank page the next time. You'll end up getting a feel for what works best, and revising your approach each time will make the cover letter more useful and relevant.

Some more resources: