Advice on Query Letters
A writer's first and often only contact, and only chance to make an impression on an editor or publisher, is the query letter.
Generally, take the time to research the publication to which you're submitting. If it's a magazine, read--or at least browse--several issues. That way, you'll avoid proposing something that's out of left field for the publication. Also, most every publications has submission guidelines that outline what they want to know. In other words, start with the basics--follow instructions. Beyond that, here are a few good articles of advice.
The Complete Nobody's Guide to the Winning Query
Title kind of says it all. Here's a nice, paragraph-by-paragraphy guide to creating an effective query letter. At the Science Fiction and Fantasy Writers, by Lynn Flewelleing