Talk to your child, reassure them, show them support, help them to feel stronger by doing this, encourage them to stand up to the bully but also walk away. Try to understand how your child feels and again reassure them that what they feel is OK, and normal. Identify the bully or bully's, try to talk to the bullies, try to make a positive impression on them, show them that you care but also that you are prepared to do something about the bullying. Identify the parents of the bully or bully's, try to asses whether or not the bully's parents would be concerned about their child's behavior towards your child. Talk to the bullies parents and make it clear that the behavior of their child is unacceptable, make it clear that you are not prepared to accept it and will take things further if necessarily. Seek assistance from others who might be able to help, remember that you might need help and support also. Try to learn about bulling so you can inform yourself to make informed choices. Never try to use bulling tactics to deal with bulling, bulling is a vicious circle and most bullies are the victims of bulling themselves.
I work with children and find that if you talk to the bullies and befriend them, treat them as real people, respect them, they are reluctant to do things that might spoil that relationship with you, so the bulling stops. Also I often find that that the bully has more problems than those being bullied but sadly the root cause of the problem is often their own parents, so having you in there life is positive. Unfortunately it is not easy to do this as a parent because it can be seen by others as being something unnatural and suspicious, so make sure that other people such as teachers and youth workers, other parents know what you are doing and why.