Besides softening agents (creams and lotions), if the skin is cracked and hard, it is no longer living tissue but simply callus tissue. That can be scraped off by one of the tools available at a drugstore for that purpose. There's a good one made by the Dr. Scholls people, looks like a rasp and is made of surgical stainless steel with a plastic handle. (There is even that "Ped Egg" thing that is sometimes advertised on TV.) The Dr. Scholls one works great. Just soak your feet, then buff or rasp the cracked, hard part. Do a little at a time, and don't rasp when you reach pink skin. Put some cream on your feet after you work on them (an antibiotic ointment like Neosporin, or else just a really emollient cream like Eucerin or Bag Balm or Aquaphor), and wear socks so the cream does not get on the rug. Do this every evening before bed, and pretty soon you'll have a clean pair of heels.
Bag balm! Sounds silly I know but it works.