One way to get children thinking about saving is to allow them to make their own spending decisions with their income from allowance, gifts, or some other source.  If they routinely blow through their allotted funds in a day or two, it won't be long before they stumble onto the idea of saving all on their own.

To prepare your youngster for the possibility of something "too good to be true" read together this Wise Pockets Parenting Activity: Too Good to be True.