Here is Brian Klems' article for Writer's Digest on submission don'ts. He's recently back from the Maui Writers Conference (apparently a great event, if you've got the means) and the five "don'ts" were collected from an agents' panel he attended. I've come across number 5 before on other blogs: "5. Don't resend a query or manuscript a day or two later with a note, "I found a mistake in my proposal and fixed it." Once it's sent, it's sent." I've never resent a query or manuscript, but I can understand the urge to try to obliterate an imperfect copy. It's funny how sometimes something can look perfect and read perfect, but once you hit that send button, all the errors seem to stand out in red to you. Don't let this happen to you! Give yourself a chance to sleep on it and then take a fresh look at your query/synopsis/manuscript in the morning and see if it's still as perfect as it seemed the night before.
Incidentally, Brian Klems' entire blog for Writer's Digest looks interesting. It covers blogging, copyrights, editors, publishing, formatting, the whole nine yards. Here's the link: http://blog.writersdigest.com/qq/ .