I think that one can find that the most effective approach to "diversity training" is to actually embed it within the company. If diversity is seen as another part of the organization, consistent with long term goals and mission of a company, there is a greater chance it will work because it is something shared by all. Just as there is a structure in charge of production, manufacturing, customer relations, there should be some structure devoted to diversity. In this light, diversity training would be along the same lines as customer service training. It is a part of the company, as opposed to outside of it. It is something that must be embraced in order to be successful as opposed to something that has to be done in order to meet a mandate or requirement.
Diversity training fails, for the most part, when it is imposed as an artificial exercise. For example, when "one day" is devoted to diversity, most see through it. The consequences to this are that it contributes to both a weakening of diversity commitments, making life more difficult for those who could benefit from a culture of embracing diversity, and helps to delegitimize authority as being artificially mandated to welcome diversity.
I would examine the episode from the show, "The Office," entitled "Diversity Day." This is a great explanation of how bad diversity training can get to be and how artificial it can be perceived. The opposite to this would be if organizations embed it within their state of being, seeking to broaden its appeal in an increasingly diverse and globalized workplace. This not only strengthens a company's approach to difference in the workplace, but helps to make the more economically viable in a diverse work setting.