[dingaling]

Drink a mouthful of vinegar, in one go. The reflex defibrilates the diaphragm.

I used to a sachet of vinegar in my daypack for that reason and it had 100% success with colleagues.

[mladenkovacevic]

That's quite drastic. If you don't like the taste of pure vinegar here's my method. I inhale all the way, then while holding my breath drink a glass of water. Doesn't that work for everyone?