Practice, practice, practice
Practice makes perfect, but do you really want to get that practice by having interview after interview -- without ever getting the job? What are you doing or saying to sabotage yourself?
You might want to consider, as soon as you begin your search for new opportunities, keeping a log of your contacts and activities. This would be the perfect place to record, following an interview, the questions you were asked, the answers you gave, and the answers you wish you had given.
Write an interview script, then get a friend to practice with you. If you're a techie, record questions on tape and practice a variety of answers. Better yet, put on your most professional attire and videotape yourself in a practice session.