by Tobias Greenich » Tue May 12, 2009 9:26 am
I know that back when I played paintball, one of the tricks we had was to soak a hankerchief/neckerchief in ice water and tie it around your neck, with the large folded part at the back of the neck. There are major veins/arteries there, and the evaporative cooling hits the blood hard, affecting most of your body. It requires refreshing after more than 45 minutes, but by then you want to be out of the heat for a while anyways.
They also sell special versions of that, which absorb large amounts of water and hold it for long periods of time. Great stuff.