Loosing is a test of ones courage, confidence and resolve.
Have the courage to congratulate your opponent on their victory, and allow them to enjoy it.
Have confidence that a loss is not the end of the world, even the greatest dart-players on the planet have bad days.
Resolve to play every future match to the best of your ability, and with good sportsmanship.
I also think that it's a good idea to remember how and why you lost a match, but don't dwell on the losses too much. If you just run into a guy that's shooting the best darts of his life, there might not be anything you can do about it. If you made dumb mistakes you can work to correct them. And sometimes you just can't do anything about just having a bad day...