I am qualified, and I will get this job. Once you arrive at the company, announce yourself to the receptionist and inform them of who you are there to see. During the interview, if you don't understand a question that the interviewer is asking, ask them to re-phrase it. Answer the question directly, without going off on a tandgem, and try not to answer with one word statements, elaborate on your short answers. Sell yourself, after all, you had the interviewer window shopping, get them to buy the merchandise.
Eye contact is another safe route to take.
I enjoy interviewing candidates that are not intimidated by my position. You should feel comfortable with your surrondings. Avoid negative answers, and as a word of advice, DO NOT bring up salary and benefits unless the interviewer brings them up first. Try to have a ball park salary range for the position that you are applying for, but wait for the interviewer to mention it first. After the interview is over, shake the interviewers' hand once again and thank him for his time.