Have them sort change, and eventually to count it. It teaches them to gather things that are alike, it's an introduction to money and it occupies them for 15 minutes! Just make sure they're old enough to be past the point of putting coins in their mouth.
Take the family on periodic “spending fasts.”
As an exercise, go a day, a weekend or a week without spending any money. Make due with what you have.
Encourage participation. Tie in spending fasts with holidays, lent or in support of a cause.
Check out the Jesuit Volunteer Corp. A short but deeply meaningful immersion into service, faith in action, selflessness and living on a lot less than you think you need.
Practice “going without.” Talk about how nobody gets what they want all the time. We should learn how to just choose to go without something we want. Get them in on the discussion and pick something every day to do without.
Misquoting someone here: “It’s not about getting everything you want, it’s about giving everything you have.” Stuff will never satisfy. Pouring yourself into something heart and soul will.
Start a yearly “review” right before school year. Raise allowance as appropriate. If they want more money, they’ll have to take on more responsibilities.