1) Be Yourself
First, and foremost, it is important to relax and be yourself. Whoever you are interviewing with is a person FIRST and a professional second. The interviewer will want to get a strong sense of who you are as a person in addition to what you are capable of professionally.
Take time to read about the company you are applying to and industry the company is a part of. Really look a the job description and understand what this company is looking for with this hire: skills needed, daily tasks, objective of the job. Ask yourself (and try to answer), how does this job fit with the broader mission of the company?
Now that you've done your research think about why YOU are the right person for the job. What relevant experience do you have? If you don't have relevant job experience, consider what educational, social, extra-curricular experiences you've had that prepared you to be the best person for the job!
4) Be thoughtful with your answers, provide context
Too often interviewees give short, abbreviated answers. There is a delicate balance of being concise but providing thoughtful answers.
5) THANK - in person and in writing
Sending a note of thanks immediately following your interview is very important. It's a professional courtesy and an opportunity for you leave a lasting impression. Always send a note of thanks!