Before you say it, think first! Does saying this help me in any way shape or form, and does me saying this hurt anyone I know?
In the case it does not help, and it's hurtful to someone, why are you saying it? Just to be mean? Or cause you're mad? Or what? Regardless of your reasoning, it's likely better for you to just keep it to yourself.
It's been a really long time since I've talked shit about someone behind their back, I think I used to do it when I was back in high school, and then when a friend did it to me, I realized how much it sucked ass. So when that happened, I was basically done doing that while everyone around me would still. I figure, let other people live their lives doing what ever they want as long as it's not harming anyone else, then who am I to say anything.
It's such a fucking petty thing to do, if you don't like the person, either say it to their face and figure out your problems, or shut the fuck up. Don't go around complaining to your mutual friends about how much an ass or bitch they are, especially when your side of the story is extremely biased.
I don't really want to go super in depth about this as it's been beaten to death by everyone, and even the shit I'm saying has been said plenty of times before, but honestly, don't talk shit about your "friends" behind their back. That is the easiest way to lose a friend.