Not so very long ago I wrote a piece advising you to take writing advice that resonates with you and ignore the rest, so keep that in mind as I toss some unsolicited advice your way. Remember salad bar, baby. Take what you want, and leave the rest (although frankly it's all good here) (yes, I'm very modest).
So in no particular order: 5 Rules for Query Writing
1. No spoilers! Do not give away plot twists! Ok, please note there is a fine art to making your book sound compelling and original, but just dumping in the big reveal isn't going to do it. I've seen those who advise an agent is a professional reader and not to worry about spoilers. This is true in regards to your synopsis, but your query should read more like a book jacket than a book report. Would you want to read a book that told you the end on the back cover? Agents may be professional readers, but they still want to get sucked in to your book by your description!
2. Look up every single agent through QueryTracker and Google. Read every interview and take notes. Did you know Jodi Reamer prefers you don't cold open your query? If you can find an agent preference online and incorporate it, you just gave yourself a leg up. Every query should be personalized in some way to the agent and their tastes and preferences, which leads to number...
3. Don't repeat back their interviews to them in your query. If the agent says on GLA that they like mysteries with exotic settings, please don't say: "I read you like mysteries with exotic settings, so I am querying you." When I say personalize I mean something a little more specific than the go-to, easiest personalization every other writer is using. A better personalization would be something like: I follow your blog and was excited to see you also loved Fantomina. I studied that novella while doing my MA in 18th century English Literature." Much more personal, right? Or if they tend to not talk about similar interests or are more formal. You might compliment one of the author's they represents work (only if you have read it!) or include something more professional such as: "I found your vlog on query tips incredibly beneficial and would love to work with you on this project." There are some Thomas Pynchon agents out there with little web presence, so do the best you can.
4. Stick to the first 30 pages or the inciting incident. When you pick up a book at the bookstore, do you just walk up to the counter and buy it? Or do you read the first few pages? It's those first few pages that draw a reader into the story, so focus on shining the spotlight on them. I passed a book in a store once and caught a few lines of the back cover. It was so compelling I stopped to read the whole cover and went out to the library to get it that night (and read it in one sitting). And all that cover copy focused on was the inciting incident: a girl who wakes up after coming home, late and drunk, to find her whole family has simply disappeared never to return. That's all a reader needs to know, and I'm guessing all an agent needs to hear as well.
5. Write a basic, buttoned down version of your query first. Get it to the point where it's a nice, average, readable query and then experiment. Don't feel pigeon-holed by rules. There's only one query writing rule: write a query that leaves the reader dying to read the book!
What are your best query writing tips?