I need some parenting help!

Gratefulness...how do you teach it? Do you model it? What about when you model it and your kids still seem to be ungrateful? Do you punish? Do you lecture?

We had the talk tonight about why we can't have it all. Yet, we have SO much! A friend of ours was blessed this past weekend with a gift. We talked about being happy for them instead of wanting what they had. Instead of wondering why we weren't "blessed" with this gift instead.

We have shown our kids people who are homeless. We have taken them with us to give out food. We have lived in Mexico for 2 months. We have modeled being frugal and thankful. We have done all we know to do and still, the ungratefulness comes out often.

Is it their age? (8 and 6). Is it our culture and society? Should we just keep doing what we are doing and pray that God will show them in time? I have heard that sometimes you might have to teach something 100 times before it is learned. I'm pretty sure we are getting close. HELP!!What are your thoughts on this?