I carry a Camelbak and fill it full of ice before adding water. You get cold water that lasts for hours, even on a really hot day. It's so great that I'll buy a bag of ice at a 7-11 and jetison the leftover. I know it's wasteful and extravagent, but it's worth it. I also carry water bottles on my frame. Better to have too much water than not enough.

Drink often! With a Camelbak it's easy to take a few sips every couple of minutes; you don't have to break rhythm and reach down for a bottle.

This summer I was riding up a really steep, long grade (it took me hours to get to the top) in hot weather. For the first time on this tour I completely drained my Camelbak, but I was almost to the top and I still had three full bottles. During the worst of it I started feeling cold and shivery. That's a bad sign - the onset of heat stroke. I was wearing a HeadSwetz "pirate scarf" under my helmet. I used some of the water to get the scarf wet and put it back on my head. What a relief! It's really important not to overheat your brain!