I heard that expression for the first time when my mom was teaching me to drive. She has a bit of a lead foot, and had gotten a spate of speeding tickets when I was a teenager. The new driver that I was, I stepped on the gas to reach the speed limit as quickly as possible. She turned to me and said, "Do as I say, not as I do, especially when it comes to driving!"
I find myself using that same expression these days as I talk to parents about reading to their kids: Do as I say, not as I do. Yes, it's true...I haven't been reading to my girls.