When I was younger me and my friend used to make ourselves pass out by breathing heavily for about 30 seconds and then putting a bunch of pressure on our diaphragm.
We'd pass out standing up in front of beds, at the tops of hills, on the trampoline, basically anywhere we could think of that might be funny/fun.
I can't seem to do it anymore, probably doing it incorrectly, though I'm not so sure it was very safe.