1. Format the Letter Correctly.
Save your creativity for the body of the letter -- or better yet, for your writing. Stick with the standard business letter format. In this example, everything is flush left, with one line between paragraphs. Unless you have letterhead, which is not necessary, type your address followed by the date. Space down a line and list the name, title, and address of the person you're writing. (Click the thumbnail, left, to enlarge this example of correct formatting.)
And as with anything you submit, use standard copy paper; type, don't handwrite; and absolutely no illustrations.
2. Address a Specific Person.
For the salutation, avoid "To Whom It May Concern." These days, most editors are listed in the masthead on the journal's site: take five minutes to find a name. Even if you're not positive you have the right person, you'll look more professional for having tried, and the letter will be forwarded to the correct editor.
3. Keep It Short.As with a job application cover, letters should not exceed one page. In your first paragraph, explain what you are sending. This can be as straightforward as: "Enclosed please find a short story, 'Choose Me, Please!' about a game show contestant with Jumping Frenchmen of Maine disease." If you have a genuine reason for submitting to this journal, share it, but only if you can do so while sounding sincere.
4. Other First-Paragraph Info.If the journal prefers to be informed ahead of time about simultaneous submissions, address that issue briefly by saying something like, "I have submitted these to a few other publications and will let you know immediately if any are accepted elsewhere." And if you've been invited to re-submit, definitely remind the editor that he or she has seen your work before.
Briefly introduce yourself to the editor. If you studied writing or have published before, state it here. If you haven't, that's fine, too. You just want to provide a context for what they're about to read.
Keep in mind that many editors use this paragraph for the "Contributors' Notes" at the end of the journal, so think in terms of what you'd like listed at the back. You can read some sample bios here, or look at some journals to see what other writers have to say about themselves.
6. Close Your Letter Politely.
Thank the editor for reading your work, and close with the standard "Sincerely," or "Best regards." Leave four lines for your signature and then type your full name. For mailing, use a business-sized envelope. If your printer can handle envelopes, type the address, but it's also fine to address the envelope by hand. Again, use the editor's name here, either above the journal name or below the address. If you put it below, write, "Attn: [Insert Editor's Name]."