I try to ride later in the day. I use a deuter hydration backpack, that basically keeps the bag away from my back - so I don't get a sweaty back wearing it. I fill my hydration bag with ice & water... keeps the water cool along my ride.

I do use cooldownz - which are basically these headband thingies that you soak in water (I use ice water) and they have some gel that swells up & retains the cold water. I just tie that around my neck.

If I do get too hot, I stop in the shade & remove my helmet and let my head cool down... if my head's too hot, I tend to bonk quickly & get miserable...