[YesBox]

I've found a few cycles of <<repeated box breathing + a long breath hold>> works quite well.

Box breathing is 4 seconds inhale, 4 seconds hold, 4 seconds exhale, 4 seconds hold. IIRC you are supposed to take in as much air as you can on the inhale, and let it all out on the exhale.

After you do a few cycles of BB, take in as much air as you can, then hold your breath for as long as you can.

You'll feel the results immediately

[meristohm]

I prefer breathing like a resting dog: deep inhale, immediately relax and exhale (just let it out; it'll be forceful at first and taper off), and take the next inhale when the need arises. Ideally I do this with water and food, too (feast/fast). I find the 4-4-4-4 contrived, but I can see the potential mental-focus benefit.