I've had this happen two years in a row. Lost two toe nails the first time and three last season. My advice would be to go see a doctor to reassure yourself that it's not serious (other than having black toes). In my case there was no pain after a couple of days and it didn't interefer with my training. Here's a link to a couple of other suggestions http://www.runnersworld.com/article/0,5033,s6-78-79-0-405,00.html But again, these are fairly agressive and I would suggest not entirely necesary. My last episode was caused by my decision to wear two pairs of socks. So maybe you could try thinner socks. Otherwise, my only suggestion would be to buy a bigger set of shoes. Hope this helps.