Be as polite and clear as possible. Try to leave little room for misunderstanding or crossed wires. Most importantly, be aware of the appropriate and imappropriate times to use email. Another problem with email is that people use it as a very informal means of communication. People simply do not take the same care when writing an email as they would when writing formal correspondence, and often say things that they don't mean.
People also assume that a message exists for the moment it is sent and read, and then discarded.
This is not the case, however. Not only do many large corporations archive email with their routine backups, but many people also archive messages for future reference. And don't assume your message stops with your original recipient. The basic makeup of email allows people to easily pass the message on to others or even seamlessly edit the text of the message with just a few clicks of the mouse. A major pitfall with the use of email is the lack of officiality associated with the medium.