If people would not change the subject line in their replies, it would be a lot easier to sort a dialogue by subject matter. So that's my rule #1: don't radically change the subject line! Add to it at the end if need be, it'll still sort properly even with that edit.

Rule #2: always sign off with your real name and a title. If my email system hasn't seen you before, and you sign off with cryptic (to me anyway) initials, I sometimes have no idea who the heck the sender is.

Rule #3: SPELL CHECK your damn email! On the Mac, I use Spell Catcher, a fabulous 3rd party utility that works in every app you type in, so as it learns new words, that applies everywhere, not just in Word or other single app. Highly recommended