Don't make negative comments about current or former employers. Think of the interview as a conversation, not an interrogation. Make eye contact with the interviewer and answer his questions in a clear voice. Work to establish a rapport with the interviewer. 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. Do not ask questions that raise red flags. Too many questions about vacation may cause the interviewer to think you are more interested in taking time off than being employed by the company. Show you want the job. Display your initiative by talking about what functions you could perform that would benefit the organization and by giving specific details of how you have helped past employers.