Here's a snippet from an Interview with Alice Pope on the SCBWI Team Blog (good advice for approaching an agent):
Would you offer some general advice on approaching agents?First, do your research. I know it's tempting to query every agent you find info on--the old "throw it at the wall and see what sticks" approach--but doing so only results in slower response times and fewer agents responding to queries at all, which no one wants. So do yourself and your fellow writers (and agents) a favor and be selective. Second, be professional but know that I'm in this business because I love the written word, I love stories, and I really do want to hear from you if our interests overlap.
Here's a Q & A from Seanchai (Author Gretchen McNeil's blog).
Gretchen: "And speaking of authors you believe in, what catches your eye in an author submission, and conversely, what makes you run screaming from a query/submission?"
Tina: "I'm most drawn in by query letters that reflect an understanding of what I'm looking to represent (you'd be surprised how many queries I get for adult political thrillers and screenplays--neither of which I rep) and that mirror the tone of the manuscript. Consequently, I'm most apt to pass on, say, the humorous middle grade novel that comes across rather, um, humorless, or the YA query that promises "a sassy new voice" but reads like a grocery list--and not a SASSY grocery list! And of course, there are the usual suspects: the mass e-query, the "Dear Sir" salutation, the incoherent plot summary, the defensive, the antagonistic..."