If you see someone being bullied, don't sit there and watch. Stand up for them! You would stand up for them if they're being victimized by a big ugly green monster, right? So why not do the same for bullying?
"That's enough, (Insert bully(s) name)?" Look at her/them like you're tired of it.
"Back off!" Same as with standing up for yourself, saying this assertivly is the best option.
"You need to stop. What your doing to her is wrong." Look her in the eye and say this firmly. If she gets defensive (and she probaly will), let her know again that what she's doing is wrong.