Behind Someones Back

So something that I have seen a lot of in my life is people talking about someone, even their friends, behind their backs. Now I am guilty of this to an extent, but I always try to think about where I want my relationship to be with that person before I go beyond saying they are annoying at that time.
One of my friends was "venting" to me the other day about another one of our friends. While I listened to everything they had to say, even the rude comments, I tried to only advise on the lighter stuff. Fully knowing that they would be friends again, which by the way they are, and everything she said would have already been said.
I am not going to tell the other person, but it's hard to see them acting like nothing happend. Some of the things that were said were too mean to just forget about.
Even though we get into fights with people, we need to try to let it play out itself and not involve other people. Now when I say I only do this to an extent I mean I do this from learning. I have had times where I have talked crap on a lot of people and now there is no coming back from that. It feels like I say something good about someone, they never find out, but when I say something bad about someone they know within minutes.
Try to be nice and whatever problems you have with someone, try to take it up with them. If you do say something to others just choose your words wisely.