First, good luck! Preparation is the key. What helps me is to speak out loud the answers to potential questions. It helps see if you are addressing question answer, and lets you hear the answer, in case something sounds off. Also, while you know the answer in your mind, hearing it/speaking it, helps gel the answer.

Second, in the interview, early on, if there is a time to say it without making it a big deal, 'confess' your poor interview situations. I find once I have verbalized my 'sins', things go much smoother.