Hypocrisy

The term hypocrisy is defined as: The practice of claiming to have moral standards or beliefs to which one's own behavior does not conform.

The term is especially used to refer to those in religious positions. In any case there are often people who preach of an ideal which they do not conform to. 
A man who preaches morality, chastity, and other virtues, but is not living them is preaching a negative message.  He teaches with his lips things that are good, but by his actions teaches that these things are simply ideals that are not attainable. He teaches that men are too imperfect to actually live godly lives. So by his actions he undos any good his message might have done. 
However, I am not advocating that we need to be perfect in order to teach people.  I am not saying that someone who has weaknesses can't preach.  None of us is perfect in that way. But what I am saying is that we need to be striving to improve ourselves. If we are working towards salvation, and continue to progress, that is a powerful example to others. If we teach with both our words and actions we will be far more effective than if we teach words alone. And the greatest person we hurt if we are guilty of hypocrisy is ourselves.