First, you must realize that you are always right. Second, you must help your SO realize this. Once she has achieved enlightenment, everyone will be happy.
More seriously, I try not to get angry at all, and begin with a purely rational approach, like asking myself “Why are you mad about this?”. Of course, this approach doesn’t always work, but I manage to recognize that many things I might be angry about are pretty silly, and I manage to avoid those disputes all together.
On the other hand, I also have a knack for making Mrs. Bubba laugh, even when she’s spittin’ mad, which tends to defuse the situation, and allow us to continue the exchange in a less heated manner.
Finally, when I’ve had all I can stands, and I can’t stands no more, I rely on silence. But I let her know ahead of time: “I have absolutely nothing productive to say. You can talk, and I’ll listen, but I’m not going to respond right now.”
So I guess you’d say that I try to keep a good sense of humor and level head. YMMV.