Don't make negative comments about current or former employers. Think of the interview as a conversation, not an interrogation. And remember, the interviewer is just as nervous about making a good impression on you. Make eye contact with the interviewer and answer his questions in a clear voice. Remember to listen.
Communication is a two-way street.
If you are talking too much, you will probably miss cues concerning what the interviewer feels is important. Reflect before answering a difficult question. If you are unsure how to answer a question, you might reply with another question. When it is your turn, ask the questions you have prepared in advance. These should cover any information about the company and job position you could not find in your own research.