Chances are, you've listened to a sports commentator and at one time or another thought, "I can do that." For instance, many catchphrases have made it into everyday vernacular, as evidenced when someone yells out ESPN's Chris Berman's trademark "he could go all...the...way" during a pickup football game. However, if you wish to become a good sports commentator, it's important to note that there's more to it than meets the eye, as much preparation and hard work behind the scenes are involved.
Prepare yourself for each game by studying up on just what your employer is looking for during the broadcasts. For example, some broadcast outlets have weekly production meetings where commentators get to meet with the players to get inside angles on what to look for in the upcoming game. Learn as much as possible about the competitors, including their stats and tendencies. For instance, if commentating on a basketball game where a team has lost a lead, it'll enhance your commentary to know if they have a history of doing so.
Write down a list of colorful terms to describe the action. It's always good advice in many aspects of life, but with good commentators, it's a science. Just like with print journalism, avoid bland words like "good," "amazing" or "very." Make a running list of things you've already said during the broadcast, and don't repeat them.
Practice at home by commentating on televised games with the volume turned down. Do this by yourself so as to not annoy your friends, unless of course they are willing to help, as practicing with an audience is helpful. This will aid you with honing your skills and improving your craft. Take it seriously; record both the game and yourself, and play it back to see how you did.
Spend as much time with your play-by-play broadcast partner as possible, as good chemistry is essential. Just like any relationship, the better you two know each other, the better you will work together. This is especially important if you're either new to broadcasting or unfamiliar with your partner. Granted, you don't want to get sick of each other, but it will help in the long run.