Several years ago a physical therapist told me to stand on a step with the ball of your foot on the step and the arch and heel over the edge. Then slowly move your heel down below the level of the step, then raise up on the ball of the foot. Repeat this several times. Whenever I start feeling any heel pain I do this little exercise, and since that time I have not had plantar faciitis.
Just be sure to hang on to the stair railing for safety.