When sending e-mail messages, be sure the subject heading is directly related to the topic of the conversation.
If you are communicating with someone about a topic and change that topic, change the subject heading as well and start a new thread.
If you are starting a new conversation through e-mail, do not open an old e-mail from that person, on a different topic, to send your new message. Instead, start a new message. Most e-mail programs will allow you to easily find your recipients address so you don’t need to use an old e-mail.