how do I teach children to manage their wants?

Often we focus on thankfullness once a year. But what if we incorporated it into a child's life all year long? We have discussions with children and take documetation of conversations we all have. It is always interesting to see how our desires change rapidly throughout our lives.
This is one discussion we have with the children regularly.

"If I could have one thing in the whole wide world, what would it be?"

"Did you pick something that will help you in life....(how)?"

We keep all the answers in a portfolio and document them a few times a year. Regularly dicussing what our wants vs. needs are help kids to understand not only the difference between the two, but realizing there are many things we are given that we love, and never had to ask for.