Christ warned against the Pharisees because they would say one thing but do another. What they said may have been true but what they did was wrong.
This applies to preachers. Those who preach are always "saying" and what they say must be true. But do they always do as they say in everything? None have been know to do so. But he must not have the attitude, "do as I say and not as I do." He must strive to practice in his own life what he preaches from the pulpit.