I have toured in the heat a fair percentage of my touring experience. A few things that I do:
- Stay hydrated! Carry enough water. Also I have found that if there are cold mountain streams it is worth carrying a filter to be able to have cold water.
- A cycling cap does help and dare I say this, so does taking off the helmet especially on long climbs in the mountains. Anyone who says it is as cool or cooler with a helmet on when climbing is full of it.
- Interestingly wrap around sunglasses that hug the face real close seem to make me a good bit hotter too. Taking them off on the worst climbs makes quite a bit of difference.
- Starting really early in the morning, like a couple hours before sunrise and stopping early or mid afternoon helps tremendously. If you want/need to get more miles in go again in the evening.