This article is from the Stretching FAQ, by Brad Appleton Brad_Appleton@ivhs.mot.com with numerous contributions by others.
With your leg extended directly in front of you, face your leg and bend it
slightly. Place your hands around the ball of your foot and gently pull
back so that you force yourself to flex your foot as much as possible.
Hold this stretch for about 20 seconds (don't forget to breathe).
Now for the isometric stretch: in this same position, use your hands to try
and force the ball (and toes) of your foot even further back toward you
while at the same time using your calf muscles to try and straighten your
foot and leg. You should be resisting enough with your hands so that no
actual foot (or leg) motion takes place.