Bg 3.21

yad yad acarati sresthas
tat tad evetaro janah
sa yat pramanam kurute
lokas tad anuvartate

Word for word: 
yat yat — whatever; acarati — he does; sresthah — a respectable leader; tat — that; tat — and that alone; eva — certainly; itarah — common; janah — person; sah — he; yat — whichever; pramanam — example; kurute — does perform; lokah — all the world; tat — that; anuvartate — follows in the footsteps.

Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Whatever action a great man performs, common men follow. And whatever standards he sets by exemplary acts, all the world pursues.

Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
People in general always require a leader who can teach the public by practical behavior. A leader cannot teach the public to stop smoking if he himself smokes. Lord Caitanya said that a teacher should behave properly before he begins teaching. One who teaches in that way is called acarya, or the ideal teacher. Therefore, a teacher must follow the principles of sastra (scripture) to teach the common man. The teacher cannot manufacture rules against the principles of revealed scriptures. The revealed scriptures, like Manu-samhita and similar others, are considered the standard books to be followed by human society. Thus the leader’s teaching should be based on the principles of such standard sastras. One who desires to improve himself must follow the standard rules as they are practiced by the great teachers. The Srimad-Bhagavatam also affirms that one should follow in the footsteps of great devotees, and that is the way of progress on the path of spiritual realization. The king or the executive head of a state, the father and the schoolteacher are all considered to be natural leaders of the innocent people in general. All such natural leaders have a great responsibility to their dependents; therefore they must be conversant with standard books of moral and spiritual codes.