You probably know what needs to go into a query letter to a publisher or producer: the name and genre of your script or manuscript, a succinct but entertaining summary of the story, and any relevant information about yourself.
Your goal is to get all of that into a one page letter or email of equivalent length. It should read smoothly and lead the reader to ask to see either the complete screenplay or (in the case of books) the first three chapters and a synopsis.
WHAT DOES NOT BELONG IN A QUERY LETTER
How rich your book or screenplay is going to make them.
That friends and members of your family have told you it’s the best thing they have ever read.
The names of several super-successful books or movies the success of which will pale once your masterpiece is published or produced.
Your complete biography (including hobbies, pets, life goals, academic achievements, favorite books, famous people you have met, etc.—unless any of these are directly relevant to the project).
That this isn’t your final draft but you think it’s good enough to give them the idea.
Summaries of another six or eight projects you have in mind.
Drawings showing the cover image or movie poster illustration you think would be good.
If it’s a screenplay, your thoughts about casting or the choice of a director.
The fact that it’s already been turned down by numerous others who obviously were not as bright as the person you’re writing to now.
The statement, “If you want to find out how the story ends, you’ll have to read the complete manuscript.” Extra points deducted for adding “lol” or a smiley face.
Even if the recipient doesn't throw away your query letter as a result, they'll consider you less than professional. And yes, every one of these has appeared in query letters to agents and producers (sometimes more than one per letter). If you avoid all of these, you'll already be ahead of the game.
(For more tips on writing--from idea through to publication or production--get my book, "Your Writing Coach," published by Nicholas Brealey and available from Amazon or your other favorite bookseller.)