(This may be a poor example, being as I wasn't alive in this era, but I think it works; let me know if it doesn't.)
I would imagine that some of the worst racism of the time happened during the Civil Rights era, because white men were afraid that black people would acquire human rights. It serves to reason that some of the most ridiculous generalizations were occurring at that time; one black criminal commits a crime, so all black people are criminals. The best way that black people fought this was to stand up and prove these generalizations wrong; Martin Luther King, Jr., with his brilliance, compassion and gravitas comes to mind.
There are a lot of generalizations being made about Christianity right now; one wingnut murders a doctor, so all Christians are murderous crazies. Inevitably, at this point in the conversation, someone declares that this is a gross generalization and that not all Christians are insane terrorists.
So why the hell haven't the sane Christians stood up and said that this is not an accurate representation of themselves? Take, for example, Catholicism (which, to be fair, serves my point the best). Most, or at least a huge amount, of American Catholics are pro-choice, pro-gay rights, and anti-child molesters; why aren't these Catholics standing up and presenting themselves as a better representation of Catholicism? Force people to acknowledge that not all Catholics are bigots.