Sports are, and should be, a joy. And it delights me that the joy I felt through the years of broadcasting games was projected onto the audience.
Don't go running off at the mouth. Let the picture tell the story. No one tunes in to hear you broadcast the game, except maybe your mother or your wife.
The single most important thing is to know the game. Study the history of the game, the fine points of the game, and the personalities of the game so you'll be able to recognize what they're doing out there and then you'll be able to anticipate certain things that are going to happen.