Be brief. If you say what you want to say succinctly, it will have greater impact.

Use descriptive key words in the subject heading. The subject line is there to help people decide whether or not they want to read it.

Be careful with humor and sarcasm. Without the voice inflections and body language of personal communications, it is easy for a remark meant to be funny to be misinterpreted. You can convey the emotions that words alone cannot express by using such online conventions as "smileys" :-)

Summarize what you are following up. When you are making a follow-up comment to someone else's message, be sure to summarize the parts of the message to which you are responding. Summarization is best done by including appropriate quotes from the original message. Don't include the entire message, since this could be irritating to people who have already read it.