blow out all the air from your lungs.
hold your breath and pinch your nose.
exhale more air (and more and more)
attempt to breath in through your nose (against the pinch)
hold your breath even longer.
(optional aerobic activity to use up the last oxygen (pushups, running))
when you can't hold any more (hold a little longer)
Physiologically internal parts will constrict to open up the airways.
Finally release the pinch and inhale only through the nose.
(airways open; ears pop, mucus loosens)
Basically hold your breath till you feel it becoming uncomfortable. Hold for just a few more seconds (but no need to pass out). A reflex will kick in and open your airways.