Get a group of friends where you constantly make (facetious) offensive remarks at one another's expense, both about individual qualities and group identifications. Eventually, having been called a filthy whatever-ethnicity or a loathsome whatever-sexual-orientation (including loathsome heterosexual, hey why not?) or a Christ-killer or a baby-eating atheist so many times, the emotional impact of such statements will be dulled, which will improve your ability to understand your actual objections and react usefully when you hear people seriously say such things. Worked for me!