Vary your intonation, use emphasis-avoid monotone. Avoid letting your voice trail off. Your energy is communicated to the class. So is your enthusiasm. Let your face be animated.
Eye contact is important.
The use of mime and the use of your hands are encouraged. Standing suggests energy but is more dominant. Sitting is more informal but limits your vision and use of the blackboard. Sitting on the desk may be a compromise. Vary the pace of the class period.