If you don't mind the feeling of wet fabric on your torso, an evaporative cooling vest is a great way to stay cool. These vests -- which construction workers also frequently wear in the heat -- are soaked in water and worn underneath riding jackets with good ventilation and airflow. The evaporation of the water helps drive down the rider's core temperature.
Don't have a vest? Wetting down a T-shirt can achieve a similar effect. If you go this route, you might not want to wear an evaporative cooling vest under anything other than a mesh jacket in order to avoid damaging your gear.