The base layers and most of my clothes get soaked with sweat pretty fast. But that's OK....water soaked clothes are more thermally conductive than dry clothes thus they wick away body heat from your skin. Just like swimming in water keeps you cooler than walking. Wet cotton is bad if its cold out but good if its hot out.
In my experience Marty is right on...
We've all been told forever to wear "loose" clothing. That is a misnomer... You don't want CONSTRICTIVE clothing. But fabrics should ride close enough to the skin to be wetted by wicking sweat from it. Once the clothing is soaked, the slightest breeze feels amazing.
I also have had good luck with the little cooling wraps with the beads in them that swell and get cold when placed in water... I tie them around my neck. It keeps the direct sunlight off the skin of my neck (major arteries and vessels in the neck) and helps cool all the blood flowing through that area.
Long sleeve shirts seem to be the opposite of what you'd want... But again, when you get hot your blood vessels dilate and are closer to the skin. Long sleeves keep the sun off your skin. And if they ride on the skin they have the added benefit of being wetted by the sweat they wick up.
Working in the heat indoors can change things up. No sun beating down, but usually no air moving either. In that case a fan is about the most effective thing going.
Mike