disclaimer: do not attempt if you have any sort of blood pressure or cardiac issues or anything that might be aggrivated by a bit of orothostatic hypotension.

First, inhale for 4 seconds, completely filling your lungs, then exhale, completely emptying your lungs. Repeat, taking one second off the inhale each time, but keeping the exhale at 4 seconds.

The effect is amplified if you bend over while doing it and stand up quickly after.