Make your calls with confidence. When you're wearing the black-and-white shirt, you are the main authority on foul and violation calls. Do not let the players or coaches dictate how the game flows--that's your job. When you decide a foul is a foul, blow your whistle strongly and make your call loudly, so everyone can hear it. Many calls seemingly could go either way (is it a charge or a block?), but you are the voice of authority.
Get to the right position. It's vital that you be in the correct spot and have a good view of the play. Depending on how many referees are officiating a given game, the positions for the referees can change. If you are the lead official, no player should ever beat you down the court. That means that when there's a turnover, you must hustle and get in your spot.
Control the game. If two players are getting into it by elbowing each other, don't let the situation escalate. If you don't call the proper fouls, things can get ugly. If you have to issue technical fouls to control the situation, do so. Don't hesitate to call a timeout and talk to the players involved.
Watch your designated spot. Don't just watch the ball. You may be tempted to follow the arc of the ball on a three-point shot or to watch the point guard dribble all around the court. Keep in mind that you should be covering your designated area, not the action elsewhere.
Ask another referee for help when appropriate. You can't make a strong call if you didn't clearly see the action. Good players will respect you for seeking the truth rather than taking a shot in the dark, even if the decision doesn't go their way.
Back up your fellow referees. Whether or not you agree with a given call, you must back up the ref who made that call. If you think he made a mistake, tell him at the next timeout, when the players are off the court. But if a clearly terrible call is made, blow your whistle, call a brief timeout and discuss the call with the referee involved.