One of my critique group members drew up a how-to sheet on writing query letters, which I then turned into a "Checklist" of elements to look for. It's often hard to edit / critique these, as each agent has different submission guidelines, which must be followed to a T.
In addition, the checklist adds a layer of distance, making it easier to point out common errors and omissions in these letters.
There are some things we just shouldn't do when querying agents. Remember, be professional at all times and, no matter how much we want to explain why we wrote the book, resist the urge to put it in the query letter.