How do you make amends to a broken relationships?

One of the problems with teens today is they do not know how to forgive. Some adolesents even hold grudges for a long time. Even if they pass the forgiving stage it is hard to forget the damage that has been done. This especially happens in critical relationships in a teen's life. This becomes a strong problem down the road when adults do not remember to show compassion or mercy for one other. Then these are forgotten and foreign concepts in some households creating a continuious cycle. Long story short, I need to make amends to some bridges I've burned down completely. Does anyone have any suggestions?