Great answer from jaded moonbeam! I can't improve on that, but I can give you one tip:

You always know what you intend to say in an interview. When you get nervous and it doesn't come out exactly as you wished, you think it sounds awful. However, the person interviewing you doesn't have ANY preconceived notion of what you are going to say. So, what comes out to them usually sounds pretty good. They accept it, or, if it's not clear, ask a clarifying question. THEY are satisfied, even if you are not. Your interview will ALWAYS sound better to the other person than it does to you. So, don't worry so much--you are likely doing better than you think!