The main things to remember when you are leaving an interview are as follows:
Do not apologize at the end of the interview.Incase you have arrived late for the interview, do your apology at the beginning,do not remind them about it when you are leaving.
While leaving the office, leave with a smile on your face even if you are feeling terrible!
Just pretend to be very happy.Make them feel that they were very patient to you. Before leaving say a big thank you to everyone. Shake hands with each of them and say goodbye to them. Look in their eyes and leave with confidence.
All this should be done even if you feel that it was a horrible interview or you found it like a disaster. It is very essential to know that there is a great relation between what you feel about your interview and how have you done in it. But in actuality ,there are very few people who can judge properly how have they done in their interview.