well if the nails are all cracked and yellowy and whatnot it might be a nail fungus...
maybe try walking around on cement barefoot, to help wear down the sole of your foot... if you wear shoes all the time then it may just be that your sole is too thick
I go barefoot everywhere I can... and sometimes the skin around the edges of my heel will get thick and crack, not painfully.
If i grind them down on the cement or with coarse sandpaper(who needs girly pumice stones
) then use a lanolin moistureizer at night it helps keep the sole skin soft and flexible so it wont be as prone to cracking the specific moistureizer i use is "Bag balm" comes in a green tin, its made for cow udders, but most drugstores have it and it works pretty well, altho it is pretty greasy