Looking for work and taking part in interviews is one of the toughest things I've ever had to do. I don't think I ever felt 100% happy with my performance and in many ways I didn't see too much difference between the last two successful job interviews I had (here in NCI in 2003, and in CBT Systems/SmartForce in 1989), and other interviews where I did not get the job. In my time I have been the interviewer on many occasions and have seen people perform great in the interview, as well as others who were dreadful.
James Caan (not the actor!) offers some tips and advice on job interviews, especially to graduates - I wish I had his advice when I finished College, I might be Professor of Zoology in a University by now? In the video below, Caan gives us "Two killer questions to close the deal on a job offer", and some other advice.
His videos are a must for any graduate seeking employment.