According to a panel of experts at APA's 2007 Annual Convention, when going out on internship interviews, be sure to:
Dress professionally. Remember that this is a job interview and you want to impress.
Be yourself. People come to interviews to sell themselves, but authenticity impresses even more than a glossy presentation.
Practice. Role-play interviewing with your friends and colleagues.
Be respectful. It's important to be courteous to everyone, from the department chair to the secretary.
Stay on your toes. It's fine to ask current interns questions, but don't let your guard down. They can pass along how you acted to the decision-makers.
Be original. Don't give the same answers to everyone at the site. Afterward, when they are discussing your visit, they will learn if you did.
Ask questions. Put your best foot forward, but at the same time, figure out if you would enjoy working at the site.