Use proper English. The language you use with your friends may be inappopriate for an interview.
Listen carefully before you answer and if you don't understand the question, ask the interviewer to explain it.
Focus on the positive. Avoid being too brief in your answers or too wordy.
Don't feel you have to fill the silence. A good interviewer knows that most people feel uncomfortable with silence and will say anything to fill it up. To avoid saying something you might regret, wait for the interviewer to initiate the next interchange.