For guys? It works pretty well, close to 100%. If you can stand up for yourself, even "losing" a fight will usually put a stop to the bullying. The desire not to get hurt, even as a fight "winner", will typically prevent most bullies from engaging in the behavior. Self-preservation is a powerful motivator. Bullies seem to try and pick most subjects to pick on because they believe that subject won't fight back. Once they do (at least with guys), they become too much of a hassle to deal with. I will caveat this though in that the subject has to be smart and assess the chances of the bully returning with a posse to even the score later.
There is a problem though with many subjects in that they are simply not physically capable to respond. I was lucky that my father taught me how to box and told me that he would stand up for me with the school administration if I ever got in a fight standing up to bullies at school. I knew, because of that, that if I was called out to a fight, I could show up at the designated time and place and give what I could. The only times I was every bullied was immediately after transferring between schools, and it only lasted the first time. However, most of my friends were not so lucky, they either didn't have the parental support, or were actively advised to keep their heads low and just suffer through. A few of them had physical problems that prevented them from getting into fights, etc. Some of my friends got hurt pretty bad as a result and were tormented throughout school.
I'm not a girl so I can't really say how to deal with girl bullying. Girl bullies seem to operate on a much subtler level than guy bullies, mostly through social ostracization campaigns. What always fascinated me is how girl bullying has almost no effect on a guy (even with other girls), so girls quickly give up on bullying guys (I'll come back to this in a second), but can devastate a girl, while the inverse, guys bullying girls can be just as effective as guys bullying guys.
It seems to me that getting a girl who is bullied out of the environment where she is bullied as much as possible, and into a group of peers where she can exist without that kind of mal-treatment (basically allow her to socialize in a safe way) can do wonders to build social skills. I have a niece who is smart, creative, talented and otherwise charming, but who is definitely not one of the popular girls. I happened to see her socializing with some of her friends in a public area when the "popular girls" came into the space. I noticed that her friends immediately withdrew while she more or less just ignored them. I think the difference is that her parents enroll her in all kinds of activities, so she has enough confidence through socialization to ignore being ignored, while her friends do no such extra-curricular activities and only know how to socialize with that particular group of peers.
The problem with girls also seems to be identifying bullying. Is she just getting ignored or is she getting bullied? With a guy, a torn shirt or bloody nose is easy to see. So I asked my niece, did she feel like those girls bullied her? Without hesitating she said "no, but they do bully my friends." And I think that the secret for her is what I said earlier about girl bullies and guys. Most guys just ignore girl bullies, and my niece seems to do the same. She has enough socialization with other kids her age who don't bully her that she feels confident enough to simply ignore the bullies and as a result she doesn't get bullied. She knows that she has a peer group she can draw positive socializations from, while her friends are still thinking that someday, those popular girls might want to be friends with them and care about maintaining that possible relationship -- which the popular girls seem to use as an avenue for their bullying.