It is beneficial to have tough skin on your feet for both ultrarunning and barefoot/minimalist running. I have tried a number of techniques for toughening the skin over the years. What is written in this entry is purely my personal experience, rather than specific recommendations. I've included a rating on how effective each approach was for me.
1 Pressure 4/5
The skin on your feet grows thicker primarily in response to pressure and friction. Walking or running barefoot on rough surfaces can produce rapid adaptation. I have found that even short barefoot walks have thickened the skin on my feet, but your feet need to be reasonably tough before you can use this approach.
2 Bathing in salt water 1/5
Salt water will dry out the skin, causing it to thicken slightly. I have found that this technique works, but the effect is very slow. Bathing the feet in warm salt solution can also be very soothing, a benefit beyond any toughening. I found using Epson salts rather than table salt is no more effective, but it does boost your Magnesium intake.
3 Bathing in strong black tea 0/5
The tannin in the tea is supposed to bind with the Protein in the skin and make it tougher. I tried it a few times, but found no real change.
4 Rubbing Alcohol 2/5
Like salt water, rubbing alcohol will dry out the skin. However, the effect is much more dramatic, producing results quite quickly. This drying effect will thicken the skin, but the drying can also make the skin crack. Note: Rubbing Alcohol (AKA surgical spirit) is either Ethyl Alcohol or Isopropyl Alcohol, with additives to make it undrinkable. Ethyl Alcohol is the alcohol you find in beer, wine, etc. Isopropyl Alcohol is different and has a higher toxicity. Both are absorbed via the skin, so I chose to use Ethyl Alcohol.
5 Rubbing Alcohol, Witch Hazel and Olive Oil 3/5
I mixed Rubbing Alcohol, Witch Hazel and Olive Oil in equal parts and applied it to my feet over night. I put it on my feet generously and then covered in thin socks. I found that this not only seemed to help the skin thicken, but it also make the skin very flexible and 'leather like', without the cracking that comes with just rubbing alcohol. Update: I have found that using alcohol hand sanitizer gel produces a mixture that applies much easier. The thick consistency allows a thicker layer, plus the mixture does not separate.
6 Witch Hazel, Lanolin and Olive Oil 4/5
I've found this slightly more effective than the rubbing alcohol/witch hazel/olive oil. The olive oil is added to make the mixture softer, and to help the witch hazel and lanolin mix.
7 Side note - calluses
A callus is a localized thickening of the skin. A callus is a bad sign, as it indicates localized pressure or rubbing, which means your Shoes are probably not fitting well. The best thing to do for calluses is to remove the root cause. I keep any calluses filed down and moisturized. I have found that moving to primal running has removed the root cause of my calluses, and they are disappearing now.