Be More Appreciative

We live in a world full of abundance. Most of us have access to everything we need and much of what we desire. Yet many people are very dissatisfied with their lives. Instead of getting into the never ending cycle of always wanting more and being critical of everyone, try a little gratitude.

Start saying thank you for everything, time, gifts, service rendered, assistance, kind words, everything.
Make a list of things and people you appreciate and make a concious effort to regularly show your appreciation for them. Add something new to this list everyday.
Volunteer. Spend time at your local elementary school, library, homeless shelter, soup kitchen, nursing home, or hospital. Give blood, be a mentor, clean up your neighborhood park. Just do something that is for anyone but yourself on a regular basis.
Know that if you are lucky enough to have electricity and indoor plumbing, try to do without it for one full day. No cheating, you still have to find a way to do laundry, cook and clean.
Understand that you regularly go out to eat, buy convenience foods, treats, even bread, or if you have help around the house, spend one week learning and doing it all for yourself. I guarantee the next time a waiter brings you a basket of bread that you had to wait a couple extra minutes for, you will feel more grateful and appreciate the work it took to get it to you.
If you have your own personal transportation, or live on a bus route, try walking everywhere for a few days.
Realize that if you find yourself being overly critical of people around you, make a mental list of their good qualities and the things they do that you appreciate (i.e. maybe your boss is not always nice, but he pays you enough to keep food on your table and most likely enough to afford many luxuries)

If you are trying to train children to be more appreciative or grateful, simply explain that you expect them to say thank you for everything. Then, when they forget, immediately take back whatever it was they were given, yes even food (although at meal time if they forget you may want to take it away for a few minutes then give them a chance for a do over)
Remember that children's behavior is strongly influenced by your attitudes and actions, so make sure you are saying thank you and showing appreciation. With young children you may even want to blatantly point things out (i.e. boy aren't we lucky the garbage men take all our trash away every week, or "wow, it's so nice to be able to snuggle down into this nice warm blanket" or, "It sure is stormy out, aren't you glad we have a home to keep us dry and warm?" etc etc)