I've dabbled in yoga but, as with so many other things, have struggled to keep up any sustained interest or make progress (but the future's bright for such things).
This though has always stayed with me. It works brilliantly and it's very easy and quick to do.
Here's my favourite instructional video on it - there are plenty more if you do a quick search of YouTube. This is quick and to the point.
For me this has proved really effective. I've been on the verge of full-on panic attacks and brought myself back with just a couple of fingers and one standard nose. It's possible to do it in public places quite discretely (I don't wear the vest for that) and I count it as part of my emergency first aid kit.
Let me know what you think in the comments.