The power of suggestion is indeed powerful. And, in my opinion, it's rarely to our benefit. Think about it -- when you have a craving, is it for something healthy like apples or spinach, chicken or salmon? Not for me.
I nearly fell off the treadmill when I heard on TV that snapping a rubber wristband against the wrist would stop the need to eat cookies. A craving for carbohydrates is as natural as feeling thirst when the body needs water.