YouTube: How To Tell People They Sound Racist
It's a few years old now, but this is great advice from Jay Smooth about keeping behavior and being separate in arguments. This advice doesn't just apply to conversations about race, I think it's good to keep in mind for any disagreement. I found this via an excellent comment by jessamyn in a thread at Ask MetaFilter about dealing with difficult people: Help us deal with our black sheep Meetup member. Lots of good advice in the thread too.