Understand the bully: Bullies don't exist for no reason. Most bullies form from outside factors, such as abusive parents or a dangerous neighborhood. Some bullies merely find amusement in seeing others suffer, so they do so in order to always be entertained. By no means does this justify a bully, but it does help to know why a person partakes in bullying.
Find people with common interests: Typically when someone is bullied, it's because they have a quirk or enjoy something that most people don't understand or follow. Because they don't understand it, they're quick to ridicule both the person and their quirk. To eliminate this problem, one can try to either expose more people to what they enjoy and explain why they enjoy it, or look for others that share that common interest with you. People are far stronger as a group than they are alone.
Is it worth your time?: Simply put, is this person worth putting up with while you could be doing... well, anything else? Why bother letting yourself be pushed around when you can do something you enjoy?