AGGEMAM said:
Only water. Anything with sugar or sugarsubstitutes will only make you more dehydrated.
Cotton or wool, is preferable over any synthetic material.
Kind of but not completely true. For me, for example, plain water races through my system. I've got to have some kind of sugar in there to get my body to start absorbing. I find that Kool-aid watered to half-strength is about perfect for me.
As for synthetic materials, polyester is plain awful, but there are some synthetics that are quite good for that type of weather.