Don't be a bully! Don't bully the weak. You are weak too. There are people weaker than you in your grade or below it, but you mustn't pick on them, as it will make you a bully.
Ignore remarks from potential bullies. If you get commented on about your appearance, personality, or another aspect of you by someone you don't know very well, just ignore them. Being ignored defeats the purpose of being a bully.
Be confident. You may be bullied for giving off an air of weakness and of self-confidence lack. Walking around with your head held high and smiling will help.
Calmly agree with their comment. For example, if you are told that you have bad acne, ask if they have some suggestion of improvement for this problem. The bully won't be expecting a response like this.
Avoid areas where potential bullies like to hang out. Diverting from places where bullies hang around can help prevent bullying.
Support those being bullied. Standing up for people who are bullied will make you more likeable by those you help, and can deter bullies; if they bully you, then they will get into trouble.
Never strike back. Giving physical or verbal abuse back is exactly what a bully wants, a reaction. Not reacting in the same manner as they bully you is the complete opposite of what they expect.
I hope these help. Here is a site that has more on preventing too:http://www.stopbullying.gov/prevention/index.html
('Seven steps' come from: http://www.wikihow.com/Prevent-Being-Bullied)