To "preach" is to tell others the right way to do something; to "practice" is to do the thing yourself. You "practice what you preach" when you yourself do something the same way you tell others to do it. Example: "You keep telling me how to hit the ball. Why don't you practice what you preach?" Our actions (what we do) should always be consistent with our words (what we say). Example: "Good managers always lead by example and practice what they preach."