I did a presentation on Effective Query Letters this weekend at the South Carolina Writers Workshop conference.
Here is the outline I used to teach the class too:
1. A query letter is a business letter
2. A query letter requires "show don't tell" just exactly like your novel does
3. A query letter MUST tell an agent what the book is about.
3a. Who is the main character?
3b. What happens to her?
3c. What choice does s/he face?
3d. What terrible thing will happen because of that choice?
4. A query letter should include the word count, the title and any publishing credits you have: Don't have pub credits? Don't worry. Don't reach either.
(the novel has to be finished. You don't have to say it is, but just know it)
5. A query letter must avoid several instant-rejection phrases:
sure best seller
"dear agent"/"dear sir or madam"
6. Things to avoid in query letters:
Don't demean yourself.
Don't quote rejection letters
Don't quote critique groups, friends, paid editors or conference contacts.
Don't ask rhetorical questions.