I think it is by being a good role model. My children are very polite (but that's from being 'taught' to be - we are polite to them (please and thank you's) so they know that nice manners are important. A lot of it comes down to the nature of the child, mine are very kind and thoughtful of others (might be because there are so many of them). This is a photo of thing 2 comforting a friend who didn't want to stay at the party without his mum -
http://tinypic.com/view.php?pic=10xtt0y&s=8#.UySw7ssgGSM
No one asked him to, he just noticed his friend was sad.