Breathe into a paper bag

Grab a small paper bag. Not a plastic bag, and not a huge paper grocery sack, but the kind of paper bag you'd carry a lunch in to school. Now, breath in and out of it for a while. The paper bag is porous enough that you won't suffocate yourself, but it will constrict the flow of oxygen to your brain enough to trick the diaphragm into operating normally. If this doesn't work within a minute, take a few breaths of normal air and try again, or move on to another method.