The great and glorious Nathan Bransford has a helpful post on how not to write query letters. He reminds us:
"Don't tell me what your novel is about. Tell me what happens."
And Janet Reid (aka the QueryShark) has a great post on Chuck Sambuchino's Guide to Literary Agents blog.
She advises queriers to have a special gmail or earthlink (or, I would assume, yahoo) account for queries with your own, real name in the address. AOL accounts are problematic because the spam block will often eat your reply. Also you don't want a shared account or a cutsie address name: Sugarpiehunnybun isn't going to get the same respectful read as somebody who sounds like grown up professional. Just the way it is. All 20 of her tips are at http://ow.ly/15XETb