All my years of horseback riding has at least paid in that aspect. Keep your heels down is drilled in to you from day one .

One good exercise that I think someone already mentioned is stand on an edge of a step with your toes and the ball of your foot on the step and your heels hanging over. let your weight drop down into your heels and stretch out those lower leg muscles.

YOu can also stand facing a wall about an arms length away from the wall. Put your hands on the wall and then lower your body towards the wall. Be sure to keep your feet flat on the floor.