Anyhow, here's five more things:
6) Check the first paragraph and the ending. I do sometimes stop after one paragraph because I can tell right there that the writer is at the writing level that would require my reshaping all their prose, even with the best idea in the world. Your beginning (and title) are your chance to hook me into reading all the way through. Make sure that you begin where you should, because often stories don't get started until the 2nd, 3rd, even 4th page. And read through that ending. Does it tie off all the loose ends and satisfy the reader? There is no room in first or last paragraphs for excessive words.
7) Make sure attachments are openable. And if they say in the guidelines not to send attachments...well then. I think we all know what your stance on attachments should be under such circumstances.
8) Remember to attach your attachment. I forget this more often than I'd like to admit.
9) Don't be afraid to query politely after a reasonable (as defined by their guidelines, usually) period of time.
10) Take rejection in stride and send them something else.
And the Ultimate Secret? Write well.