When trying to empty a
bottle quickly by holding it upside-down, the
main factor which slows the
flow is that
air has to rush into the bottle to fill the space previously occupied by the
fluid. This periodically halts the flow of liquid from the bottle, and is the reason for the
glugging sound heard when pouring too fast. Squashing the bottle to squeeze the liquid out will obviously alleviate this problem, however this isn't feasible when the bottle is made of glass. Shaking the bottle up and down, or banging the base of the bottle while pouring, have little effect on the
speed at which it empties, since the "glug factor" will still interrupt the flow.
There is, however, a simple technique which will enable you to win the bottle-emptying races common in certain entertainment-starved communities. Invert the bottle, then quickly swirl the whole vessel in small horizontal circles (i.e., the axis of the bottle remains vertical at all times) of a few inches' radius. You should only need to perform a few of these circles before the liquid leaving the bottle forms a funnel shape--it will stream out of the bottle only from the rim of the neck, leaving a channel in the centre through which air can flow upwards into the bottle, thus neatly solving the "glug" problem. The speed increase will be marked, and your place in speed-pouring history assured.
Other methods include opening the bottle outside the Earth's atmosphere, and (quickest of all) informing a group of university students that it contains pure alcohol.