If you're a writer, still in the 'aspiring' category, then bookmark this post by author JM Tohline about the biggest mistakes a writer can make in a query letter.
JM emailed 100 agents asking one question: What is the single biggest mistake writers make when querying you?
He has responses from over 50 of them, and he's compiled them into this very helpful post. Definitely worth saving the link for later use. I know I'll be needing it one day.