The main way I teach this "giving' and "serving others" value to my children is through stories and example. During our family nights in December I try to find lots of stories about "giving." I tell the stories and ask the children what we learn from them. Then after the stories we do some sort of project that will help others.
I think the most important part of these nights is to ask them the question, "How did this make you feel inside?" after the family service project. Then I explain to them that giving to others makes us happy. Our best Christmas was one year when each child decided to give up their best gift to someone who we knew was going through the hard time of losing their father. The kids still talk about it, and that night feels very special in our memories.
Throughout the year I also try to do many things for others and to let my children see it and participate if it is approriate. I used to make the mistake of "giving" to others while my children were at school. But then it occured to me that I needed to let them see me doing things for others.
This year we are going to use this idea from premeditatedleftovers.com. I'll let you know how it turns out!