Like lots of people, I tend to live in world of a gazillion emails and have a reasonable system in place for handling the flood. However, this post at 43 Folders reminded me that I haven't put as much thought into sending mail. I think it is well worth a read.
Amongst the particular things I already like to do with sending emails is clarify the subject of any replies I make. For example, if I'm sent an email with the subject of "Team Meeting", I will modify it to "[Tue 3pm 290505] Team Meeting". Also, I ensure that I always address the person by name, and put my name at the bottom. Makes it so much easier when reading a long email that has gone back and forth.
Now, I'll consider prepending "FYI: " or "RA: " or "RI: " to the subject of every email I send, as suggested in that post. Perhaps the first line of the body should be a legend - "(FYI=for your information, RA=request action, RI=request information)"?