I'm surprised no-one has mentioned this before but I will, because, as a recording engineer and guitarist, it is a technique I use and teach to others: hyperoxygenate. That's right. Spend a while breathing deeply to saturate the brain and tissues with oxygen before you start a take. Besides the benefit of causing you to breath more shallowly for a while, it will also relax you and give you a feeling of well-being. If you need to, stop down for more breaths and pick up again. I've been known to stop and do it just before the last measure so that I don't spoil the last ring-out with my humanity.
Bob