Thanks for a valuable post, Maggie. Could I add another suggestion to your own advice? Before sending out your query letter, folks, study this blog and its archives:
http://queryshark.blogspot.com/
It's run by an industry pro who doesn't pull her punches in advising would-be authors on their proposed query letters. I wouldn't mind betting that she's hussled many of her readers through the right doors.
A fantasic place ... but don't submit your proposed query if you're thin-skinned. She's some tough lass and tells it like it is.
Cheers. Neil