In 1990 while I was a student at Southampton University I did several firewalks organised by the Wessex Sceptics, a group consisting mainly, as far as I can remember, of Physics postdocs.
My first and second walks occured after they gave a lecture on firewalking, the main thrust of which is that firewalking has nothing to do with mind over matter and everything to do with thermodynamics. Wood embers can be very hot, several hundred degrees celcius, yet they conduct that heat very poorly. This means that if you put your foot on the embers you would have a second or so before you'd feel any heat. Of course, this means you can walk quite safely across (assuming you keep a steady pace and don't fall) without getting burnt. On the other side of the pit you have to ensure that you knock off any embers that have stuck to your feet, otherwise you get blisters.
The photo above is of my third walk across the embers, on my very last day at the University. The photo was taken by my dad.
Firewalking gave me a real buzz, mainly because experience tells you it's not possible but science says it is. Before you walk across you can feel the heat of the pit on your face and it takes some faith to start walking, but once I went across I felt such a high for having done it. Well recommended.