Good sportsmanship

How do we teach children good sportsmanship? I reckon it is true that we can't be good winners till we can be good losers. This means I am not above beating a 5 yr old in a race. I discovered this a few years back, when I would let the little boy I was caring for win and he was downright insulting afterwards. I decided I was not going to stand for this so I won. I ran at my full speed and he was left far behind. He may not have been left so far behind if he had continued running, however as soon as he saw I was in front, he stood and cried. Screamed actually. I don't remember if I went back for him or not, however I told him he should have finished the race. I was also teaching him the correct verbal responses to win or lose outcomes. These were "Good race" and "congratulations"

Over a few weeks, I continued to win races and then would turn around before he could have a tantrum and cheer him to the finish line, we practiced congratulating each other. I don't remember how this ended but if I was intelligent, I would have let him win to see how he reacted. If he was insulting again, we would have gone back to square one, if he was gracious, then he had learnt the basics of good sportsmanship.

Now I tell children that winning is not as important as finishing the race or game. This helps them if, in a competition against peers, they don't come first. I tell them I am proud of them for finishing. However bad sportsmanship may not even be as clear as it was with the little boy described above. It may just be a smug look, or grizzles and tears over not winning. The same strategy should still work.
Also I am not saying we should crush their hopes by winning by miles every time, just bring them to earth in humbleness. Then, when it is time to let them win again, allow them to pip you at the post, this will give them the satisfaction of a hard won success and will keep them humble in their victory.