Be confident.
Research the company you're going to. Knowing about them gives you some insight and leverage. It also helps you understand their culture a little bit.
Know your worth. Meaning: Don't let them lowball you. If you've done your research on salary (which you should), you know when they are giving you bullshit offers.
Arrive 15-30 minute early. Listen to music to calm you down beforehand. This also gives you time to observe the environment you'll potentially be in for a while as a fly on the wall.
Turn your phone OFF during the interview. Obvious reason here.
Don't be afraid to point out disrespect. If the interviewer is being disrespectful of you, your uncertainty, or your time, do NOT be afraid to call them on it. Also, feel free to stop the interview and walk out, if needed. You'll probably never run into this, but eventually, it happens.
Don't be afraid to ask for the job at the end.
Don't be afraid to say "No" to them.