Feivel,
This is what I found; the requirements to give Tochachah are:
1) You have to first assess that there is at least a reasonable possibility of the person listening to you. (Sometimes there are Halachic ways of assessing this.)
2) You have to give the Tochachah in a non-aggressive manner, and never in front of people.
3)You have to make the person understand that the only reason you are giving him the Tochachah is because you care about him, and it is for his good, so that he can get Olam Habah.
It also says in Sefer HaChinuch perek 239 that you should give someone tochacha privately and in a nice way; but if they don’t listen to you, then you should embarrass them in public so that they will do teshuvah.
(Please correct any errors you find from me.)