The night before the game, I like to quickly check my notes and make sure I've got all the major bases (which are belong to us) covered. They usually are, so I enter the land of nod with nothing but enthusiasm for the next day's game. But in the hour or two before everyone arrives, I sometimes have to put down little anxiety rebellions over some aspect of the notes, be it their order, clarity or completeness, or over my familiarity with rules I expect to come into play (one of the best things about 3.x D&D is that there are aspects of the game I have yet to experience in play).
However, once we're all ready, I don my +6 mantle of self-confidence and get stuck in. In my experience, being able to purvey an air of self-confidence helps to keep a game moving and improving. I always do a session post-mortem the next day, to see where I went wrong and to examine what I might have done better. But if - no, when - I share a self-critical observation with one of the players (between games), the response is usually supportively dismissive of my perceived failing. That's nice, bearing in mind that my adventure design and DMing are frequently described in complimentary but nevertheless somewhat harsh terms.