How? Sit or stand (with your hand on a wall or chair for balance) and place a tennis ball under the arch or one foot. Roll the ball slowly, back and forth, from heel to toe for two to five minutes. If you find a tender spot, slowly apply pressure (whatever feels comfortable) until the tightness releases. Repeat with your other foot.
Performing this little massage routine every day or two will result in feet that feel better as well as looser calves and hamstrings.
Looking for other handy alternative uses for tennis balls? Check out this retro Tennis Fixation post: Recycling Tennis Balls.