Heat acclimatisation helps a bit, so I make a point of doing rides even when it starts getting uncomfortable.
On top of all the great suggestions already made by others, I wear long sleeve jerseys even in summer (for UV protection) but I soak the sleeves in water whenever I fill up my bidons from a tap or soak them from any other water source. The evaporation helps quite a bit. It takes about five times more heat to evaporate 1 kg of water than to melt 1kg of ice and bring it up to room temperature.
I also take a small towel with me and soak it in water. It helps to remove salt from the face and it can cool my neck.
Last edited by joewein; 07-14-20 at 08:26 PM.