Interesting. I can't comment on the parasympathetic vs. sympathetic responses; however wanted to tell you that I attended a stress workshop here at work the other day, and we practiced breathing techniques.

First, we tensed and relaxed our muscle groups, starting with the feet and working our way up. Then, we breathed in deeply to the abdomen, through the nose, for a count of four. Then held the breath for four, and exhaled for eight. Repeated this several times - probably a dozen.

We were told that this is good practice for relieving physical symptoms of stress.