ANDY'S ANSWER: The first thing is not to participate. The second thing is to speak up when someone says something inappropriate to or about someone else, eg, on facebook or some other social media. This includes the things that teens post under the guise of being funny.
A lot of teens think that the jokes they make to one another on a public forum are just in fun, but in fact they can be embarrassing or hurtful. One thing I see all the time is that "friends" will highjack someone's account and post something potentially embarrassing or even damaging to the person's reputation. In my mind, this is akin to being assaulted. It doesn't take long for joking to cross the line to bullying, in the eyes of the victim anyway.
Considering how someone may feel about what is being posted BEFORE posting it would go a long way to solving the problem. Peers who make it socially unacceptable to treat others in a manner that is less than respectful would eliminate it.
Below is the recording of Andy answering this question.