Here are two breathing techniques I've been using lately. The first one is from Dr. Andrew Weil, it's simple, and it's easy to remember.
Here's Andrew Weil's 4-7-8 breathing method:
That's all you need to remember, actually. Breath for a count of 4, hold for a count of 7, then breath out for a count of 8 to, in Weil's words, "reset your involuntary system."
I don't know if that's what's happening when I do the exercise, but I can say that every time I do it, I feel better.
The second technique is from a cult-like figure named Wim Hof.
Here's Wim Hof's method, in brief:
Take 30 breaths, ballooning your chest.
Breathe in as much as you can, expanding your chest.
Breathe out completely, drop your chin, and relax until you feel your reflex.
Inhale again deeply and hold for 10 – 15 seconds.
Reserve five minutes at the end to scan your body.
Here it is in a little more detail, if needed.
Get comfortable with legs crossed and prepare to warm up.
Inhale deeply until you feel some pressure in your chest. Hold your breath, then exhale, pushing out all of your air. Hold for 2 – 3 seconds.
Repeat warm-up 15 times.
Once you're warmed up, pretend to blow up a balloon by inhaling through your nose, exhaling through your mouth in short, powerful bursts with a steady pace. Repeat 30 times. If you get lightheaded, that's OK.
During your big breaths, focus on each part of your body. If you sense something that doesn’t feel quite right, release that part of the body.
After you’ve completed your 30 power breaths, inhale deeply and fill your lungs to capacity, then force the air out. Drop your chin and relax. Hold your breath out until you experience the gasp reflex. Then inhale normally.
Inhale again deeply. Release any tension in your chest and hold that breath in. Drop your chin to your chest and hold for 15 seconds. Scan your body again for anything that feels “off.”
Relax and scan your body.
I've been using the 4-7-8 breath at small random times, like on the subway or in a meeting. I use the Wim Hof method when I need a deeper "intervention" -- like if I'm overthinking something at bedtime or in the middle of the night. Give both a try. I would love to hear how it goes for you