Exercises for posture start with the feet as it's essential to build on a firm and sustainable foundation. The arches of the feet are the dynamic bony springboards that absorb your weight and propel you forward.  Feet make a big difference to the attitude of your body in space, all the way up - even to the way you hold your head and balance your glasses on your nose.

If the arches of the feet roll in and flatten there will be less dynamic support of the body. It will also make the feet dullards in the role of push-off during walking – making you stump along, hauling one leg after the other, instead of prancing forth like Nijinsky with a spring in your step. 

The tibialis muscles of your lower leg run down either side of your ankle, overlapping under the foot as a beautiful elastic stirrup to sling up your arches. You can see this described and a diagram in my Ankles Chapter of 'Body in Action' combined with some really effective self-help exercises to get your arches back working.

Just as important however, is the attitude of the feet on the floor, since this greatly affects the ability of the arches to function properly. Walking with the feet turned too far out, like Charlie Chaplin say, is just as bad as walking pidgeon toed because it also affects the knees.