Some thing we used to do road racing(motorcycles) is to put our hydration pack in the freezer the night befor and then wear it under your clothes. It works great and you still have water to drink since the temp. out side and your body will melt it. Lately what I have been doing is wearing a cap soaked in cold water under my helmet. Every so often take it off and soak it with the chilled water from the hydration pack.
Slainte

p.s. and still have a pint upon returning home.