Don't be afraid to say, "I don't know." If you don't have an answer for a student's question, tell that student that you'll find an answer and get back to him/her soon. The instructor who thinks that teachers should know everything is going to have a much harder time than the one who accepts that teachers are living, breathing, humans who are subject to error, like everybody else. In addition, students can usually tell when you're making something up, so you're not saving any face by providing answers. Rather than lying, tell a student that you're unsure and that you'll get back to them. You'll find that they tend to respect you more for being honest.