This breathing helped me many times.

You can perform it in any positions, in a shop or on a meeting.

Keep in mind that you will have a very short time to breath in, so be quick with the inhalation. You'll have to keep pressing the tongue up on the palate in the meantime. 

Breath in to 4, hold the breath to 7 and exhale to 8. 

After couple of cycles go back to normal breath.