Three simple improvements that you can make to yourself are to dress well. Dress not for the position in which you are applying, but for the position overseeing that one you applied for. A great outfit will give you confidence in yourself.
Carry yourself well and stand with poise. 65% of communication is nonverbal; make your body language work for you.  Stand up straight, smile and remain confident. Use only a firm handshake when introducing yourself and exiting the interview.
Listen intently to the interviewer. Understand what is said before you respond. In an interview it is important to listen intently, comprehend fully and respond accordingly.