Klim makes the ultimate hot weather helmet.
Also conditioning makes a big difference. Make sure you are well hydrated and eat plenty of salty food leading up to the ride.
I've used flexible ice packs on my back with a soft back belt to keep my core temp down. - https://www.amazon.com/JellyBeadz-Therapy-Cooling-Bandana-Scarf/dp/B004XX4XA8 - Keep these in ice cold water, throw one around your neck, and you will stay cooler longer. I ride in 100 degree temps in the woods here in Texas, and this is what I do. I can ride for 20 minutes or more before I really start to feel the heat. Without it, I can't even last 20 minutes in the heat.
I found these to be cheap and effective. http://www.amazon.com/JellyBeadz-Therapy-Cooling-Bandana-Scarf/dp/B004XX4XA8