Now that you've read the good, the bad, and the ugly of queries, I have one last suggestion . . . query critique groups.
I've made this suggestion at numerous writers' conferences and I will bet that no one has ever followed through on it (if you have, please let me know, I'm curious how it works). Just as everyone has a critique group or critique partner, every single one of you should have a query critique group. This group needs to be completely separate from the one you use to read your book and needs to be made up of people who have never read your book. The idea is that when you read each other's queries, you aren't looking as much for grammatical errors as you are to see if your letter really grabs someone and makes them want to read your book—in the same way book cover copy should grab your attention and make you want to buy the book. This is much easier to do when the person doing the critique doesn't already know the ins and outs of your book.
Having a query critique group will help you narrow down your book description to one paragraph—the one that needs to be exciting, eye-catching, and make an agent want to drop everything and read!