I feel strongly that a large part of what I must do as instructor is to prepare my students for the work force. Oftentimes, my students are remiss interacting with classmates, and with me, with the proper amount of respect and professionalism. So, I try to always model proper work place communcation, e.g., more formal interactions. The trick is remaining approachable and jocular while presenting myself as a figure of authority. Almost always, responding to an inappropriate email with the proper style and type of remarks helps to redirect a misguided student. A good example is still the best teaching tool in many instances.