I followed my nose yesterday afternoon and found this guy, J.M. Tohline, and he wrote this unbelievable post on the BIGGEST MISTAKES WRITERS MAKE WHEN THEY QUERY.
You need to read this whether you've queried before or not. He polled 100 agents and got their responses to what makes and breaks a query letter.
Go here now.
And then, when you're done there, go to his follow up post at the end. It's definitely worth the read.
That is all.