Get that thick, dead skin off first! Once you do that, you will heal nicely.
I have tried everything mentioned in these responses and nothing worked. (My heels used to bleed in the afternoons the cracks were so deep.)
I finally found a Dr. Scholl's hand-held, metal foot scraper (I don't know the offical name for it - it kinda looks like a mini cheese grater. It has a white handle and is about 8" x 1") and used it on my dry heels. I have found using it on dry skin works best.
I basically sandpapered off all the thick, dead skin until I got to the nice, soft, new skin. (It took about an hour of hard scraping the first time!) Then, I started lotioning my feet at night and wearing socks.
It was sooooo nice to have "real" feet again!!! (And, the cracks felt SO MUCH better! They didn't hurt as much and they were able to heal without that thick layer of dead skin surrounding them.)
I still get "hard" heels occasionally (my feet are still at the bottom of my priority list. LOL) but my Dr. Scholls buddy does the trick every time. (I have gone through 4 of them in the last 2 years.)
Good luck and enjoy a pretty-feet Summer!