At Nationals last week, I sat in on several agent panels and gleaned lots of information. Today I'm going to tell you what they said they are looking for in a query letter. This info would also work for queries sent to editors.
They all said keep it professional: Like a job interview, keep it pithy and to the point. Of course they all said nearly the same about the query letter, but all wanted different amounts of the actual work sent to them.
One said you use this method: Hook, Book, Cook. Which means- Hook, a one to two sentence hook about the story. Book, no more than two paragraphs about the book- like a back cover blurb. Cook, a brief bio (publishing history, awards) about you and any specific reason for writing the book.
They said a query should never be over a page long. An agent or editors time is valuable to them and they don't want to wade through a two or three page letter. Most said if it is longer than a page they won't read it. Also if you have met the agent or editor at a conference, do mention that at the beginning, but don't go on and one about the meeting. Again, be pithy and to the point.