Learn first. If you have not already received a degree or certification in theater arts, learn as much as you can. Study with different coaches and teachers and in different genres of drama. Become accustomed to the act of performing so that you can give your future students well grounded advice and guidance.
Use games. Teach drama with impromptu games and interactive group activities. Playing can be good for a child and an adult, and it helps to free the mind and the body to react naturally. Impromptu games are can help actors to create realistic characters. Having the entire class "be a tree" is not as helpful as having them play out unscripted scenes drawn from a hat.
Continue to learn. Teachers are qualified to teach their subject, but continue to learn after you begin teaching. Stay abreast of the latest trends in theater and movies, and this will give you new and different ways to prepare your students for theater and for life. Teach drama as you would teach any other subject by reading and studying drama constantly.
Discuss interesting topics. You don't necessarily have to delve into the human psyche in every class, but discussing different motivations for a persons actions as a class can help shed light on scene work and monologues. Find out what you individual students enjoy outside of class and pick scenes and study works that will cater to those interests. Especially in a school setting where drama may be a requirement for a certain degree, try to draw your students in that are taking the class because they have to and not out of an interest in the class.
Teach from experience and from texts. Using well established actors and teachers to help you convey a message is not frowned upon. Require your students to study Stanislovsky, Meisner and other important acting schools of thought. A well rounded drama student will be more successful in dealing with different scripts and future coaches and teachers.