I have the same issue as Laurajbr but not from sewing. I've had no success with the exercises she does. What my chiro recommended was to put ball of foot on a step and stretch heels downward. I'm terrified I'll fall doing this, so I just pull the ball of my foot toward me and stretch out my leg as far as the tendon will allow. If I remember to do this on a regular basis, I have no issues.