Know if the agency/publisher wants a query letter only or the first three chapters and synopsis and send it. Though, Miss Snark (http://misssnark.blogspot.com/) says to always include the first five pages with your query so he/she can get a feel for your writing.
Evil Editor critiques query letters. He's so funny I'd like to marry him. I'm not brave enough to submit one of my letters, but you can learn a lot by reading his critiques. http://www.evileditor.blogspot.com./
I've heard several editors say they hate it when a writer tells them their book will make them laugh, cry, etc. Then, they're waiting for you to prove it. They also don't like writers to compare themselves with well-known authors because usually the writing comes up short of that comparison. Let them come to their own conclusions.
A query is your calling card. Above all, be professional. Make it as perfect as possible. Don't be cutesy or put yourself or your writing down. Keep it to one page. Don't send it by email unless the agency lists that as a viable option. While you're waiting, start another manuscript. If a publisher/agents lists six months as the time it takes for a reply, wait longer than that to inquire about your status.
Bribery doesn't work. They may enjoy the chocolate you send, but it won't make them buy your book. Also, no emotional blackmail, i.e., my kids will starve if you don't publish me.
One editor once said brightly colored paper would get her attention. However, it's best to stick to the basic white or buff. Don't use your company's/office's letterhead.
Here's my suggestions for a query letter:
Your address, phone number, and email. You want them to be able to find you.
The editor's/agent's CORRECT name and address. CHECK SPELLING! They hate to have their name misspelled or be called Mr. when she's a Ms.
If you have met this agent/editor before or have a recommendation from their client, put that up front. He/she may not get to the bottom of your letter. However, don't invoke a client's name without their approval. It will be found out and will only make you look unprofessional.
Boil the manuscript down to a couple of paragraphs. Hit only the high points (goal, motivation, conflict). Don't keep any secrets. Mention only important characters (hero/heroine; protagonist/antagonist; villain).
Your writing credentials. If you've published before. If you've won awards. Some editors/agents like to know of any contest finals. Some like to know if you've achieved RWA PRO status; some don't. That's something you won't know unless you hear that agent/editor speak at a conference or have a conversation with them. Impertinent information wastes their time. For example, that fact that you're a vet isn't important unless your character is a vet.
Book title. The genre/sub-genre. Word count. Whether it's complete or not. (Most editors/agents only accept queries for incomplete manuscripts from the previously published.)
Include a self-addressed stamped envelope (SASE). You may not hear back otherwise.
Sincerely, Your Name
Have you found any tried and true methods you'd like to share?