One factor parents have no control over is how a child is treated socially by his/her peers. Those who are not welcomed into "the in crowd" basically have 3 options.
1. Hang out with other kids (in their same league)
2. Live a solitary life with no friends.
3. Become a bully hanging out with other bullies demanding respect/fear from all the other kids.
Some kids get raised to hero status after beating up or defending themself from a bully. Now he/she is the new "king of the hill". It's very easy for them to get swallowed up into a power trip after that.
Since we have no control over other kids and what goes on in their household all a parent can do is make sure they have a close and tight bond with their children. The kids have to know they can talk to you about anything without you automatically getting upset. Most kids who are bullied never tell their parents. It's not until after there are obvious signs.
Parents have to assume the responsibility for their child's welfare. Telling a kid to "work it out on his own" or simply saying, "fight back" is not always the answer.
For those parents who have children that are bullies probably the best thing you can do when they are young is show them where most bullies end up. (In prison or in the grave).