Hard skin occurs as a result of excessive pressure or friction of the foot against the ground or shoe. This can be due to the heel rubbing against the edge of flip flops or sandals and also high heels.
Using a file (emery board), gently remove the hard skin, clean it and then moisturise with a vitamin E based moisturiser.
Sometimes the skin may be too thick and hard, and in such cases its best to seek professional help.
However, in some cases, such as if the problem results from deformation of the bones or joints of the foot which can lead to a biomechanical problem, then using an orthotic can help and in some very extreme cases where the problem is persistent and painful, corrective surgery may be required.

There is also a condition called pitting keratolysis, in which the sole of the foot has shallow crater-like depressions in it, giving the impression that the skin is hard and needs to be filed to smooth it out. But, as the condition is caused by a bacterial infection (nothing to do with excessive pressure or friction), filing won’t help much. Actually, the treatment for pitting keratolysis is simple, requiring only care with personal hygiene and the daily use of a bacterial soap, until the skin returns to normal. Your clinician will be able to advise on the most appropriate soap to purchase.