I wandered around on a few web sites trying to figure out how to write a good query letter. Here is a list of the tips I gleaned on my journey.
-Structure your query like your story. Unfold it for the agent or editor
-Don't put your punch-line / reveal right up front
-Entice the recipient into reading your novel—that's the whole point
-Read it aloud to make sure it flows well and sounds good
-Make sure you have rhythm in your sentences
-Take out everything that isn't needed—extra words and flowery descriptions
-Including the character's motivations is key. What does the character want?
-Make it clear what the stakes are your novel
-What choices need to be made by the protagonist?
-Show, don't tell . . .
-Who is the antagonist?
-Don't send a list of “ingredients” (a boring outline) send how to bake your cake
-The entire query letter should be right around 250 words
-Show the voice of your story in the query letter
-The level of polishing and writing in your query reflects on what is expected from your novel
-Include the word count and genre at the end of the query. Title of book should be in all caps
-Leave out personal details about yourself unless they somehow pertain to the agent or (and better) the story. If you're a nurse and the story is about a hospital note that. If you're a nurse and your story is about flying pigs, probably doesn't matter.
-Use the agent's name when you query
-Make sure they represent what you're trying to sell them
Telling the agent if this is there are planned sequels for this novel is up for debate. Some say yes, others say no.
I'm still working on my query. There never seems to be quite enough time in the day . . .