Emergency breathing techniques? Don't even think about it. As indicated on another currently running forum, it's the Oxygen partial pressure (or lack of it) that's the killer. With certain breathing techniques you may be able to increase the air pressure SLIGHTLY, but to nowhere near the degree necessary.
Holding your breath? At 40,000 feet depressurised, to maintain Sea Level pressure inside your lungs would be a 12.0 PSI pressure differential. That's enough to burst any aircraft pressure hull.
Oxygen is the ONLY answer, and at higher levels, at 100% and under pressure.
Regards,
Old Smokey