POTS pro tip - if you need to stand up or stay standing and your blood is going to pool in your feet, squeeze all of the muscles in your legs, HARD. It will help keep blood from pooling by forcing your blood vessels to constrict. The Air Force trains their pilots to do it so they don't pass out under high g forces, so it works. If it's not working, try doing it sitting until you feel like you've got the hang of it. If it still doesn't work, at least you tried.