I agree with what has been said thus far. From my perspective, I would give this advice to any referee: Keep your eyes open, your mouth shut, and study the rules!
"Keep your eyes open." Many referees and judges simply don't pay attention enough to the action. They look sleepy in their chair, they watch other rings, or just don't keep a sharp awareness of what happens so they can award points (judges), or assess penalties (referees). You have to look at the center of action and use your peripheral vision to take in everything, and remember who did what to whom.
"Keep your mouth shut." Too many judges, and referees get themselves in trouble by talking to athletes, coaches, parents, or each other about match results. According to WTF rules, this is a big "no-no." Do not discuss anything with anyone except tournament officials (IE: arbitration board). Don't tell anyone "you should have won that match," or "the other judge didn't score fairly." Keep your comments to yourself, and don't lose your cool. Just smile and say "good job!"
"Study the rules." If you know the rule book front to back, you will be able to apply them properly durning a match, and avoid having your decisions officially "protested." People will sometimes "grumble" when they lose, but they will be less likely to challenge you if you look professional and confident, and you know the rules. When you are questioned by an arbitration board, you should be able to give sound answers as to why you made a particular judgment call which coincides with the rule book.
Safety is of the utmost importance. Fairness should be automatic (if not, you should not be a referee). The Referee is there to help manage the match, and encourage a smooth, uninterrupted display of the athletes talents. Any competitor who attempts to gain an advantage by violating rules should be penalized appropriately. Be fair - be consistent - and be solid and confident in your commands. Practice, practice, practice. Go to many seminars, and referee at many tournaments to gain experience.
Good Luck!
Chief Master Eisenhart