There was a show on the history channel (back when they actually had shows about, you know, history) that had a scene in it showing a choreographer instructing someone on this subject. Basically he had someone attacking another person with a sword from behind. The person being attacked was to lift his sword up over his shoulder and pointing down his back, which deflects the blow.

Obviously that's a little hard to animate properly if you're not doing a cut scene, but the point is that it's not that far outside reality.