The Guide to Literary Agents website includes a guest blog by Ricki Shultz. The article focuses on agent Janet Reid and her 20 top tips on query letters. In addition to "Keep it brief" and "remain professional", Janet includes "Surefire Query Killers" and what I think is the best advice of all, "Remember: You can query too soon. You cannot query too late."
Check out the article and make those query letters work for you.