If I have a problem with how someone behaves--
I tell them if it is severely impacting the relationship and I want the relationship to improve.
I tell other people who know them if it's bugging me enough that I want to let off steam and those people are handy, or if they mention similar problems with the person's behavior.
I would rather that my comments not get back to the person, and sometimes I ask the people I'm talking to not to tell the person, but I don't particularly expect them to do as I ask.
In my experience, people relay information along friendship-closeness lines. If I am medium-close to someone and I tell them something about another person that they are very close to, I expect them to tell the person, because they're closer to that person than they are to me.
I don't know if my prevention mechanisms work - I haven't had a lot of people coming back to me angry because of things I've said about them behind their backs (tho' it's happened once or twice), but there might be people who've heard about my behind-the-back comments and haven't said anything to me.
If I have a partner commitment with someone, I try not to tell other people about problems in our relationship that I haven't also discussed with them. I'm not perfect at this - if I have difficult communication with someone, I have a bad habit of avoiding discussing problems with them even if the problems are affecting our relationship. If we have good communication, though, I tend not to do this.
If someone has a problem with how I behave, I want to be told if we have a close relationship and it's impacting our relationship.
I don't particularly want to be told under other circumstances - I prefer relationships to be based on "take me as I am" on both sides. If I am told, I try to respond with sensitivity, but if I'm told in anger and if I feel demands are being made of me, I get defensive and don't behave in a very sensitive manner (tend to withdraw).
I think people do say things behind my back that are different from what they say to my face. I think it's inevitable, so it doesn't bother me that much. I don't do anything about it.
wild_irises and elisem made good points about compliments also being passed around behind people's backs. I do this too. I sometimes tell people if someone else has complimented them. I think I take compliments that are passed via a third party more seriously than compliments that are given directly to me. That's weird, isn't it?