Kids are bombarded with messages about how important they are, and how they should always have what they want. It's OK to have some stuff, but advertising and other marketing messages in today's culture can make them think it's all about them. It can lead kids to believe the axis of the world runs through the tops of their little heads.
Think about this. In 1971, the average person saw 564 advertising impressions a day. Now, that number is about 4,000. The purpose of advertising is to disturb and influence you to the point that you'll buy something. Advertisers want you to believe that you're not complete without their product, or that you'll be a happier, cooler, better person with their product. And in most cases, advertising and marketing people are more aggressive in their teaching than parents are in theirs.
My suggestion is to find some giving exercises in which you can all participate. You could adopt a single mom at your church. Make it a family outing, and go buy groceries, gifts for her kids or even a Christmas tree. Make sure your kids are involved physically, mentally and emotionally in the entire giving process. Let them experience the grateful, and sometimes ungrateful, responses that go along with giving. And make sure you do some things that don't involve money. You could take the entire family to help cook and serve dinner at a homeless shelter.
One of the best things we ever did as parents with our teenagers was to send them on mission trips. It truly changed their lives. When you see real poverty close up, when you live and walk and sleep in it day after day - I'm talking about death-and-disease poverty, not the American version - it changes your heart. And when you're 13, it will change your life forever.