I have a feeling that however much we deny it, we all have secret prejudices. It might be gender or race, or it might just be short people or balding men with combovers. Consider your opinions of people, and see if there might be a pattern based on an identifiable characteristic. Whether you think women are bad drivers or people with southern accents are stupid, there's probably some unjustified opinion you have of someone. And I think identifying it is the first step toward eliminating it (or at least harnessing it for your own amusement).