This came up the other day when my fiancee and I were out on a ride. It was hot and she commented on how nice some cold water would be.
I'm sure this isn't a secret trick, but it's one I picked up in the Marine Corps. It worked wonders in Iraq when I'd be out on long unsupported ops in the desert. Take a sock (clean if possible. if not, oh well), stretch it over your water bottle and soak it in water. When it's hot outside, the phase change of the water evaporating from the sock will cool the water in the bottle dramatically. I've never used a thermometer to test the result, but I'd say you can easily take water from 90F+ ambient temps down well into the 60s, maybe cooler.
It works great and very quickly on the bike because of the air flow. I find that little things like a swig of cold water can be a great amelioration of mental stress when you find yourself really wearing down.