Socks, people, socks. When I run races in the winter (which the ones I have done so far are all city races) I wear socks, because my feet are not used to running on that damn chip and seal and when it is cold, my soles conform even less. And I don't like to carry sandals either. And socks are closer to barefoot than sandals. Though, at some point during the race, holes do wear in the bottom of them. Next time I do this, I am bringing two pairs of socks. The soles don't just get tougher, they actually get thicker too. I never followed the 10% thing cuz it really depends on what kind of surface you are running on, for how long, and for how fast, and how good your form is -too many variables for me to deal with. I just did what felt good and that varied a lot per day initially. My "base" run has improved over time, but at it's own pace. I have been running barefoot for almost a year (in July) and my "base run" on pavement was at 3 miles for about the first 9 months, and is up to 4 miles now on both pavement and trails. My soles are distinctly thicker and trail running really made them thicker, faster. I will run through any water I can get my feet on.