I breathe deeply and quickly through my nose with my whole torso (rather than just my belly, a mistake I carried along since teen years) to warm myself. I don’t have near the purported abilities some do, but what I can do so far I find useful. It can also help me go to sleep, as maintaining such breathing takes all my mental bandwidth, at the expense of rumination.

I recommend reading Breath, by James Nestor.

I also recommend only taking cold showers, because they feel great afterwards and because they save water and heat energy. “Breathing up” beforehand helps me bite the bullet if I’m feeling reticent.