We always assume that "we" and "our" group are always better than every other group. I often considered why we hold such bias. Part of it might be rooted in how we view the world based on our biological need to be in a protective environment to keep our genetics safe. While that is a possible explanation it doesn't seem to hold up against idea that entire groups that can contain millions of people where natural biology has no place. To understand bias is to understand ourselves and our own needs.
I find that people who are biased often discuss the "other" in terms of fear. Fear that they are doing something wrong, will take something, or will cause some havoc. Their focus is limited to a few examples and seem to raise the perceived validity of those arguments beyond their natural merits.
Beating bias through critical thinking means we must perceive the world around us as accurately as possible. That means not ignoring dis-confirming information. We must accept the information as it comes to us while ensuring it is unadulterated first-hand information versus that which is related by others.
We must think through all of the possibilities both for and against an argument to come up with an accurate assessment. When new information comes along we should continually update our cognitive models in order to ensure it is accurately portraying what is happening. People prone to bias simply won't accept counter information and won't adjust their models.
If you view why bias people hold onto their beliefs is because it is tied deeply to their self-perceptions. They feel that if they accept the inherent quality of others they are lowering their own value. Trying explain that the "other" has similar value creates an affront to their self-esteem. Exposure, environment, and time can make a difference only if the person is open to experience.