Know the rules. Every sport has its own specific rules and you need to know them backwards and forwards. All sports that use a referee should have an official rule book. Some rules may change according to the level of play you are officiating at. Make sure you take this into consideration.
If you are using hand signals, make sure they are visible to the players on the field. Use eye contact to make sure the timer and any other officials are ready to resume play.
Voice calls and whistle blows should be loud and clear. Buy a new whistle every year. They do wear out.
Be professional the moment you step on the field. Everyone is looking to you for a fair game. Your appearance, conduct and attitude set an example for players as well as coaches.
Be fit. Physical fitness is essential for referees as well as players. Especially in sports such as field hockey, the referee must be in position to call a play. This often involves a sprint up or down field. If you are lagging behind the point of play, it is impossible to make an accurate call.
Work as a team with the other officials during an event. If another referee has a problem, take care of it without involving the players or coaches.