I didn't have cracks in my heels so much as I had callouses and skin buildup on my big toes and the area right beneath them, plus all around my heel area. It looked white and dry and was really hard.
I used a foot file and scrubs, which helped, but it didn't fix it. Finally, I thought I've got to get rid of all that built up, dead skin. I put my GlyDerm lotion (which I normally use to help exfoliate my face) on my feet, waited for it to absorb, then put on the Bath & Body "Shea It Isn't So" lotion, and socks. I did this every other night for a week or so.
It really got all the dead, calloused skin off so I could put lotion and socks on at night and keep them looking great!