We began when they were young. When they wanted that expensive yogurt they saw on the Disney Channel, we'd explain why were were purchasing a different brand and the benefits of the savings. Kids as young as three can understand that a few bucks saved on yogurt translates to something more fun later.
We also encouraged (and still do) alternatives to spending. My boys have more Legos than the average big box store. When they see a new, expensive set they desperately want, their first instinct isn't to beg for it, but to sort through their own Legos and try to recreate it with what they already have. When my eldest wanted a new game system recently, he rounded up all the games he no longer played along with an ancient, no longer used system and traded it in for the new one. He went a step further and traded for a used refurbished system, so his out of pocket cost was only $15.
My husband and I also believe in transparency. By the time my kids are 10, they are active participants in the household budget. They know just as well as we do where our money goes, and they sometimes offer their ideas on saving money in a category.
I drive one of my sons and two of his friends to their weekly Scout meeting. Recently the other two boys were going on and on about a new video game that my son hasn't played yet. He mentioned he had the money to buy it but hadn't yet. When his friends asked why, his simple response was, “It'll be available used and half price in a couple of months. I'm going to wait until then so I don't blow all my cash.” I love having kids that are already confident enough to set goals, spend smart and ignore financial peer pressure!