One of the best ways to avoid common instructor mistakes is to be well-prepared. I find that when I have a detailed lesson plan, reviewed the answers to all the assignments, and have prepared all lesson materials, I am more at ease in the classroom and less likely to fumble or make mistakes. Another way to avoid common mistakes is to chat with your colleagues and share your mistakes and how you overcame them. Colleagues are great resources and support--especially for a first-time instructor.
