How a child deals with bully like behaviour will help effect how much bullying they are likely to be a target of. Responding to bully like behaviour in a "CALM" way can help shut down bullying early and prevent them from becoming a target.
CALM in this case stands for "be Calm, be Assertive, Look them in the eyes and Mean it" and can provide a basis for teaching children how to interact with a bully.
Be Calm - Bullies feed off reactions. If they are able to get you worked up or get a reaction out of you they are successful. Picking on someone that doesn't get a reaction is noun to them and they are more likely to move on. When you get nervous you tend to talk faster, so practice speaking slow and be mindful of how you talk.
Be Assertive - Bullying is a power game, it's an attempt to knock someone else down in order to feel in control and powerful. Being assertive shows them that you are still in control.
Look them in the eye - similar to being assertive, bullying is about balance of power. Looking someone in the eye is one of our primal power plays. Practice looking someone in eyes, staring contests are a common children game, but they also teach how to appear confident.
Mean it - Don't say things you don't mean, and mean what you say. If you say "back off or I will do X", it needs to be something that you can follow through on. If it isn't, and they call you on it you lose credibility.