Do always smile as you meet the interviewer/HR/your potential new boss. Who wants to work with a grouch!
Always shake hands when meeting and leaving, Research has proved that it leaves then with a warmer impression of you.
Make that handshake firm , but not an Arnold Schwarzenegger impression.
Be open in your answers. A good interviewer generally should not be asking questions that can be answered with a yes or no, but they might be still learning.
Don't you just hate don'ts! Don't let's have any!