I have mentioned this before but it bears repeating (at least I think it does ;) ). When I started p-90x it kicked my butt (and still does actually) and when I got to the Yoga routine included in the program I thought..."Oh great, a break" Man was I in for a surprise. I did 1/2 of a 90 minute routine before I was toast.

ST, I want to emphasize the breathing suggestion I am advocating is to breathe through the nose but forcefully exhale through the mouth. Repeat until you feel settled. Often it might just be 3 or 4 loud exhalations before I feel settled (still proceeding with the activity BTW).  repeat as needed.