Start small with quick rewards. Let them earn a bit of money for a week and start saving it in a jar/piggy bank/bank-account. Take them to the store and let them buy something. The next time, let them save for two weeks. Explain the difference in their options than the previous time. Work it up until they're saving for months at a time, and allow them to come to the realization that the longer you save, the more you save, the more useful it is in the long run!