Answer
Walk around barefoot a lot
More input
Flatten the arch of the foot
Put on weight
Get older
Get pregnant
Massage your feet as often as possible. The loosening of the muscle and ligaments relax your feet and relieve the strained tension, thus putting your foot (and body) at ease. You can actually feel your feet stretching.
An additional suggestion
Move up to high altitude, your feet swell when you're pretty high up.