In order for a person to avoid becoming the victim of a bully, he must have developed the internal belief that he is acceptable. Those who don't have this belief really struggle to defend themselves
In Aikido it's part of my training to help my fellow students develop this awareness within themselves, therefore I encourage them, accept them as the unique individuals they are, and smile a lot
People who are surrounded by discouragment and disparagment will naturally come to think of themselves as unacceptable.
It's up to us as parents, teachers, relatives and friends to ensure that those around us get the right messages from us.
It's also up to us to recognise that about 100% of the disparaging remarks made towards us are not aimed at us personally
(Scarily enough, I have only learned all of the above fairly recently in life and I am in my late 30s