OK, so it's an odd topic to come up with in the middle of actual editing, but it cropped up and it seemed to me it was one of those topics that are never really out of mind. After all, without a decent query where would we be, writer-wise?
Publishing Crawl have been showing sample query letters, and commenting on them succinctly and sensibly. Neither fulsome praise nor harsh criticism, just commonsense advice. So it seemed equally sensible to pass on the gist, in terms of a short, easy-to-apply checklist for writing a query letter to agent or publisher.
1- make sure you greet the person you're writing to. Yes, obvious, but people do forget it's a letter, not a synopsis.
2- give your reasons for sending them the query. If you don't have one you haven't done your homework?
3- give the title, genre/age range, word length, and standalone or series up front. That way the reader can instantly determine that the book in question is one they might take on. Not a fiction book when they only take non-fiction. Not a romance when they specialise in horror. Doing daft stuff like that not only wastes everyone's time, yours included, but makes you look a right fool?
4- In one or two paragraphs, relate the main characters, the plot, and what's at stake.
And then stop, without falling for the temptation to add just a bit more, and a bit more and....
This made sense to me, so I hope it's helpful to your thinking too. :)