I never thought of the DM as an opponent of the players. He is (or she is) rather, a storyteller. Too many DMs get so caught up in the desire to "conquer" their players that they forget the reason those people show up in the first place. Invite disbelief, don't enforce it. Encourage your players, don't railroad them. And most of all, challenge them, but don't frustrate them. This is, after all, a game.