One almost sure way to fall out.. when they start taking your blood and they give you the little ball to squeeze, have a competiton with your best friend to see who can fill there blood bag up the fastest

You would be surprised what people in basic training would do who are addicted to sugar. I was in a squad with some guys who gave blood just so they could eat the cookies afterwords ) Unbelievable that people would do that, but it was pretty amusing, and for a good cause.