Know the material:

The purpose of teaching is for students to learn. To help a student who is having trouble understanding the material, you need to know (a) where the student is having a problem, and (b) how to explain it differently the second time. Both require that you know deeply the material you are teaching. It is not good enough to be "one page ahead" of the class, or to have memorized the material well enough to write it on the blackboard without making any mistakes.