Little mean girls: Helping your daughter swim in those choppy social waters — Washington Post
This article has some decent tips about preventing social ostracism, but I would caution to be careful about generalizing all mean behavior as "bullying" True bullying is repeated, it causes harm, and happens within an unfair match. Techniques are: isolation, dehumanization, disempowerment and inevitabilty.
That may feel beside the point for the parent of a child who is victimized at school, though. Carol Kaufmann offers some approaches to help kids navigate social choppy waters.
"It’s also important for girls to understand that instead of watching bad behavior from the sidelines, they can be part of the solution. 'Activating bystanders and getting them to intervene in an appropriate way is a key to reducing bullying,' says Catherine Bagwell, a professor of psychology at Oxford College of Emory University"
This one is well worth a read, with a comment from Katie Hurley, author of No More Mean Girls: The Secret to Raising Strong, Confident, and Compassionate Girls.