Along the same lines as exercising your feet, you should also stretch them on a regular basis to keep the muscles, tendons, and ligaments in your feet and lower legs healthy and supple. The Achilles tendon that attaches the heel bone to the calf is particularly prone to injury when it becomes tight and is overused, causing Achilles Tendinitis to occur.
A good way to stretch this and other tendons in the back of the lower leg is to stand on a step or raised platform with your heels hanging over the edge. Drop your heels below the edge of the step while keeping your toes on the step. You should feel this stretch down the back of your lower leg.