Darts is considered a "gentlemanly sport". There are certain things that are expected of players, scorekeepers and spectators. While there are some players that do not follow these guidelines, it is usually because they are rude, ignorant or sore losers. Better players will not like to associate with these kinds of players, and often such activity can lead to the better players giving the offenders a "taste of their own medicine". Stick to these simple rules, and you will have a much more enjoyable game and make a lot more friends than enemies.
Sportsmanship:

Generally, all dart players are good sportsmen. They will shake hands at the beginning of a match and wish each other good luck. Similarly at the end of the game, they will shake hands and say "good game" or "nice shooting".
Score Your Darts Before You Pull Them:

This can avoid many arguments. If you write down the score on the board before you pull the darts from the board, you can show your opponent what you threw if there is a discrepancy.
Don't Be A Sore Loser:

Appreciate what the other player has accomplished. If the other player bested you, live with it. Remember if you had thrown something really amazing, ally amazing, you'd be happy about it ,so be happy for the other player too.