I ran across Query Shark while web surfing the other night and stayed up way past my bed time reading blog posts. If you send an agent query to the blog, the Shark may choose to post and critique your letter, and believe me, you'd be lucky if she did. While there are a few glittering jewels, most need some work and their failings are learning opportunities for all of us.
The Shark's advice works for every stage of writing. First, voice is everything. Then, know who your character is, why she is interesting, and how that drives the story.