One of the greatest gifts haShem has given the Jewish People is the ability to cleanse ourselves of our sins through Teshuba (repentance).
When we speak Lashon haRa we cause untold damage to ourselves, to our listeners and to the subject of our words. We should try repair the damage, at least to some degree.
If one has spoken Lashon haRa but his listeners did not believe what was said, then the sin is one between man and God. Teshuba in such a case requires just that the person regret his sin, confess it before God Almighty and accepts upon himself never to repeat it again.
If, on the other hand, his words were accepted as fact and it resulted in harm, then more is required.
For example: A person lost an opportunity for a promotion because someone provided unnecessary or inaccurate negative information about him. This constitutes real damage, both monetary and emotional. In this case the three-part Teshuba outlined above would not be sufficient. One would also have to approach the victim and ask forgiveness for having spoken against him and caused him harm.
Certainly, this is a very difficult thing to do, especially, for example, if the victim had been unaware that he was being considered for a promotion. Nevertheless, neither YomKippur, nor death itself, can erase a sin between man and his fellow man unless sincere forgiveness is sought and granted.
(The founder of the Mussar Movement, Rav Yisrael Salanter, found difficulty with the above stated rule. He suggested that if by telling a person that we spoke negatively about him, we will cause him additional pain and distress, then perhaps it is better not to inform him).
Adapted from : “Chafetz Chaim, a daily companion”.