The Gemara teaches that one must reprove over and over, even a hundred times. It may be unwise to tell someone bluntly how wrong his actions are, for it will probably antagonize him. How, then, do we understand this statement from the Talmud which requires us to reprove our fellow a hundred times?
People abhor criticism. No one enjoys hearing that what they're doing is wrong. In order to give rebuke in the correct manner one must have a proper strategy. By telling us to rebuke even a hundred times, our Sages are teaching us how to give rebuke: one should break up the criticism into a hundred small parts. This way a person can draw the object of rebuke closer to his point of view in a palatable way, one step at a time. Only in this manner will the person be able to hear that what he's doing is wrong, and thus be willing to effect a change.
Additionally, it is important to have the right attitude when giving rebuke. If your intention is to set him straight only because it is bothering you, your rebuke will almost certainly fail.
A Rabbi once told over a story about a student in the Hafetz Haim's yeshiva who used to smoke on Shabbat. All the guys would scream at him to stop, but it was to no avail. One day he got a message that the Hafetz Haim wanted to see him in the office. There was a big commotion in the yeshiva - the Hafetz Haim had found out that he was smoking on Shabbat! The student went into the office, and came out two minutes later. After that, he never touched a cigarette again. No one knew what the Hafetz Haim had told him, but in two minutes he accomplished something that throughout the years no one else had been able to accomplish!
After the Rabbi finished speaking, an elderly man came over and said "I was the boy that was caught smoking on Shabbat." The Rabbi asked, "So what did the Hafetz Haim tell you?" The man replied, "When I was called into the office, I started to tremble; I didn't know what to say. I came into the office and there the Hafetz Chaim was. My feet started to shake. He motioned for me to come around the desk. I came around and he held on to my hand, not saying anything. He then looked at me and just said two words slowly: 'Shabbat Shabbat.' As he spoke, two tears dripped down from his eyes on to my arm, which pierced a hole right through my heart! He wasn't just telling me off, like I felt everyone else was doing. Rather, I could see that he really cared about me and the honor of Shabbat. I decided right then and there never to smoke on Shabbat again!"
This is the way one gives rebuke. We need to analyze our motives before giving reproof and work on really feeling for the person.
Rabbi Isaac Farhi