The test Haidt mentions you can take to test your implicit prejudice about race, gender, sexual orientation and other topics. From the Project Implicit organization, which educates the public about hidden biases.
This article in Personality and Social Psychology Review (2001) provides an overview of negativity bias (the human mind reacts to bad things more quickly, strongly and persistently than to the equivalent of good things).
Haidt refers to studies in this journal article by Cialdini in which he found that concession leads to concession (college students saying no to working with juvenile delinquents for two years, but yes to chaperoning them on a trip to the zoo). He found the best way out is to fight reciprocity with reciprocity.
In discussing reciprocity, Haidt writes, "the elephant is a natural mimic." He cites the experiments in this journal article (2003) which showed that waitresses who mimic their customers get larger tips.