OK, it is going to sound silly but it works for me. It's something I learned in a workshop 40 years ago.
Sit or lie quietly with your hands at your sides. If you are sitting, let the arms hang loose.
Take a deep breath in through your nose. Now, exhale through your fingertips. That is, as you exhale, focus on your finger tips as if the air was leaving your body through the tips.
Do this several times and see if it helps. It works pretty reliably for me.
2013 German Vasquez Rubio Concert Special "Bernardi"; German spruce/Brazilian rosewood
2014 Garrett (Gary) Lee spruce/cedar double top; Brazilian rosewood