someone has mentioned salt, and if I remember right, for long distance walks, people used to apply white spirit or something, not sure about the health side of the latter.
Extract from the web: quality unknown
) For a while now I have been soaking my feet just once a day in cold yet very strong tea. I haven't wasted any teabags, I used the ones left over from the day. Already the skin on my feet has became slightly tougher. I wouldn't swear by this technique yet because I have only done it for a week or so.
2) Surgical Spirits. I haven't tried this one yet but apparently some hikers swear by this. CAUTION: : If you have sensitive feet, they may bleed.
3) Walking barefoot on steadily hardening/sharper ground can slowly help toughen your feet. This is generally like conditioning - it takes time. Take this technique easy and progress cautiosly. Don't go running through glass in your first week.