> Nothing has been shown more effective at tearing down prejudices than actual exposure and confrontation.

The result has been fairly inconclusive. What happen is that people generally keep their views about in-groups and out-groups, but then add exception for the person they get exposed to. A good experience/friendship do not translate to a change in definitions for the in-group, nor does it change existing negative generalizations of the out-group.

What has shown to be effective is demonstration of shared values by the out-group, while at the same time avoiding display of different values. When people share the same values, and more importantly, do not display a difference in values, then the in-group can be expanded.

If I remember right, the book Behave by Robert Sapolsky goes through this.