Do your homework - Recruiters expect you to research and learn more about their organization. Thoroughly review their web site and talk to current (or past) employees of the company to learn as much as you can. Being prepared will also minimize your anxiety. ReadyPrepInterview is a site that provides a very solid list of common interview questions, searchable by job title; check out their database of job interview questions here.
Make an impact - Dress appropriately. When in doubt, dress conservatively. Recent workplace trends have approved business-casual attire, but it's still appropriate to wear a conservative suit to an interview. How you present yourself makes an impression. Shake hands firmly and maintain eye contact throughout the interview. Sit up in your chair and stay focused.
Be confident - Composure in the work place is crucial. Your interview is a measurement of how you handle pressure. You don't want to appear too nervous. At the same time, you don't want to be too relaxed. Maintain an appropriate level of professionalism without being unapproachable. The best advice is to be yourself. If you are an outgoing, likeable person, let that come through in your interview. Recruiters are looking for colleagues who will thrive in their team-based environment.
Ask questions - Asking questions demonstrates your interest and preparation. Before the interview, prepare a list of questions to potentially ask in the interview.
Sell yourself - Your qualifications got you in the door. Speak confidently about relevant experiences you've had in the workplace, during volunteer opportunities, and in the classroom. Specific examples of how you've contributed to an organization or learned something new will be of particular interest. Your goal is to demonstrate how you can help the company meet its goals and objectives.
Get clarification - If you don't understand a question, ask the interviewer to repeat it or for clarification. You have a better chance of giving your best answer if you know exactly what was asked. If you can't think of an answer at that moment, ask to come back to the question later.