I'm answering because I slightly disagree with the other answers. The argument their making seems to be that the more confident one is the less their bully, I don't agree, or actually I do agree, but I believe increased confidence has only a very very minor role on tendency to bully.
In truth I think personal confidence and tendency to bully are mostly independent factors. Having volunteered with and watched many children it seems by far the most common reason for bullying is an attempt to make others like you, as bizarre as that may sound at first glance. Social politics are complicated, and sadly it often is the case that bullying someone that is unpopular can make you popular with others, or at least a certain type of person. However, bullies are not primarily driven by a sense of confidence, or inferiority, but instead by a desire to 'show off' to their peers. A sense of inferiority can exasperate this, by making them think the only way to impress their peers is to bully others, but many bullies do it without a sense of inferiority, and many bully who are not overly confident.
So the short answer is just build the child's confidence and don't worry about bullying :). But I'm not good at giving short answers, so lets instead ask what can you do in general to prevent a child from bullying.
First and foremost, as with most behaviors, model good behaviors. Never let a child see anything in your actions that could suggest bullying. This doesn't just mean don't call someone a poopyhead when your kids are around, there are subtler forms of 'bullying' adults do often without realizing it. The most common is how you treat people in customer service industry who have to be nice to you. If you treat them with respect, say please and thank you, and if you have a complaint explain it civilly to them, that shows a respect to someone in a weaker position, if you yell or rant at them your suggesting it's okay to mistreat those who can't fight back.
Going along with that showing children the importance of general respect and empathy with others will help to prevent bullying. Help them to discuss other's feelings (as well as their own) so they can understand how other's feel. If they do something to upset a peer when young have them say their sorry and explain why what they did could hurt the child's feelings and let them know you don't want them to make others feel sad.
It also helps to try to expose the child to many different types of people, with different views, behaviors, and lifestyles and show them that even though the people are different you still respect each of them; that someone being different isn't a reason to treat them poorly. Even if someone does something you don't approve of or makes you angry try to explain to the child that you disagree with the person's decisions but that doesn't make them a bad person or worthy of hating. That means you have to show civility to people of the opposite political spectrum as you no matter how stupid you may think their views are :P
Since bullying is often driven by peer pressure and the drive to do anything for popularity helping arm a child with the means to resist peer pressure will also help them resist the temptation for bullying. Of course helping a kid resist peer pressure isn't easy, that's a whole different stack exchange question. A big step to this is to encourage your child to be different though, or more accurately allow your child to be different. Parents are, in a way, the first form of peer pressure children run into. If parents pressure a child to be 'normal' and fit in so the child doesn't embarrass the parent their teaching the child they need to do what other's expect of them. Encouraging a child when their different, and not being afraid to act different yourself, helps. Generally explaining the idea that if someone wants you to act in a bad manner to make them like you then they aren't the sort of person that they probably want as a friend or should care about liking them in general.
And if all else fails, well just flat out tell them you won't condone bullying. If you see it happen you punish them. If you see even minor forms of bullying, like talking about a kid being weird behind his back or tolerating a peers bullying, tell the child how disappointed you are to see it, why it's wrong, and that you don't condone it.
Honestly though. all that comes down to good parenting advice in general. If you raise a child well they usually won't be involved in bullying. Just show them through your actions how to be compassionate individuals and they will grow up the same.