Practice, practice, practice! Check out this list of the 50 most common interview questions and then find a friend to role-play with you. Have your friend act the part of the potential employer - and find someone who will take this seriously and really try to put you on the spot. Then you enact the "interview scene" several times with a variety of questions from that list - different sets each time. Make eye contact with your pretend interviewer, smile, be honest, and find out which questions trip you up the most. Practice this way with a real person who is safe and when faced with an actual hiring manager, you will feel much more comfortable and confident with your answers.
My second suggestion is to do your homework on the organization. Look at their web site. Google the company for stories and news items about them. Figure out who does what, which department is having a banner sales year, and what the press is reporting on. When you are in the interview, you will be given the opportunity to ask questions. Pick something that you discovered in your research and ask something about that. For instance, you see that the company just won a major civic award in the city. Figure out (ahead of time) a good question related to that award - how did they get involved with that project, for instance. And then ask that question instead of asking about time off or salary. The interviewer will be impressed that you know something about the organization - not enough people take this step.
Finally, dress professionally, shake the interviewer's hand and smile when introducing yourself, and thank that person for their time and consideration when the interview is over.
When I teach business writing, I always have my students follow this advice and it has served them well. Remember that you want to present yourself as someone that they want to hire. So think about what you might want to see in a potential hire if YOU were responsible for hiring someone. You want an employee who is confident, prepared, courteous, willing to answer any question, and who has some basic knowledge about your organization. Sounds ideal, yes? And if you take these steps, you will set yourself apart from your competition.
Best of luck!