Practise Yourself Before You Begin to Preach by Swami Sivananda

A certain old man took his boy who was addicted to eating much black sugar to a Saint and addressed him, “O Santji, my boy eats much black sugar. Kindly advise him to give up eating black sugar.” The Saint said, “Come after fifteen days.” The Saint himself used to take much black sugar like the boy. He gave up at once eating sugar for fifteen days. He found no trouble or mental disturbance. When the old man came on the fifteenth day with the boy, the Saint addressed the boy with great force, “My dear boy! Give up this habit of eating sugar at once.”

The old man asked the Saint, “O Sadhu Maharaj! Why did you not advise the boy on the first day?” The Saint replied: “I was myself a victim of the sugar-eating habit. How could I then be able to advise others? I gave it up for fifteen days. I corrected myself first. How can I preach to others when I am myself filled with Doshas?” The advice of the Saint had very great effect on the mind of the boy. He gave up eating sugar from that day.

The moral of this story is that you must put a thing into actual practise yourself before you begin to preach. Then only it to others. will it produce a lasting impression. Example is better than precept. It is easier to preach to twenty than to be one of the twenty in following the preaching. 

(pgs. 129-130, Yoga in Daily Life)