Signatures are automatically added to you outgoing messages. Include your name and affiliation. Do not type in capitals. This is the e-mail equivalent of shouting. Keep humor to a minimum.
Implied humor is often misunderstood in e-mail.
Much e-mail today is casual, however you still should watch your grammer and spelling. Be concise, the best messages are less than one screen long. When you respond to an e-mail, include appropriate parts of the original message. This will act as a reminder to the original sender. If I am asked a question in a message I always leave the question and type my answer just below.