It's amazing how much pouring water on your head helps. Also soak your clothes with it, particularly your shorts, to keep your thighs cool.

For hydration, they typically say a big sports bottle an hour (24-ish oz.?), but in extreme conditions more. You have to figure out for yourself how much "more" is for you. Better to err on the side of caution. Once you start feeling symptoms of dehydration, it's hard to hydrate and feel good again.

Electrolytes are very important, of course.