I agree with being consistent, though that isn't always easy to do (especially when grading). Here is my advice:
-don't be afraid to say you don't know. Students can ask some good questions, and instructors/TAs aren't expected to know everything. Just say you're not sure and promise to get back to the student. Then, actually look up the answer and respond to them.
-students are not your friends. The pull to want to be liked by students can be strong, but (teaching evals aside), friendship isn't your job. Don't Facebook friend them, block students on gchat, and don't be afraid to say no (check with instructor first, of course)
-you know more than the students! You've gotten to graduate school, these students have not. Trust that you know where you've been. You know more than you think you do.
-the students are more afraid of you than you are of them. Remember, you have some power here (not ultimate power, but some). You will be grading their exams and papers, not the other way around.
-speaking of ultimate power, being a TA is pretty nice because you have some say over the students, but you can punt back to the instructor for any difficult questions. Follow the syllabus and what the instructor tells you. If the student has an issue with that, they can take it up with the instructor; you didn't design the course.
-especially if you're nervous, dress professionally. I firmly believe that clothes make people feel differently, and can help you feel more in control. If you normally wear jeans or sweats, wear a skirt or trousers to teach. Wear professional shoes instead of sneakers. I find that dressing professionally makes me feel more adult, which is useful when you're only a few years older than the students you're teaching.
-be yourself. You got past the graduate school interview, and you have friends. That means people like you. Not everyone will, of course, and that's OK, but some do, and just allow some of your own personality to come through. If you like to tell jokes, tell jokes to your class, but if you don't tell them in your "real" life, don't try to force it.
Those are my tips and tricks for today!