As a general rule, you can't teach a lesson that you haven't embraced yourself. If you throw a hissy fit every time you lose, it's not fair to turn around and ask for good sportsmanship from your children. You don't need to go all-out positive and congratulate yourself for losing, either. Just make an effort to acknowledge when someone else wins, and adopt an optimistic attitude.
Source: Flickr user MissTurner