Quick-drying, high-wicking t-shirt, sun sleeves and a neck-gaiter.
I started riding with these a couple of years ago and when kept wet, they really keep me cool up to 100F, which is as hot as it officially gets here in Colorado Springs , except my bike thermometers often show up to 120F in direct sun, which at 6500' in altitude, is very direct!
The short-sleeve shirt with sun sleeves works better than a long-sleeve jersey because air can blow up under the short sleeves and cool you off around the arm-pits and upper chest.
I wear the quick drying neck gaiter bunched up so it holds more moisture and cools through both shade and evaporation.
And as with the separate sleeves, air can get in the top of the shirt, unlike a turtle-neck jersey.
I just bought a white helmet for this summer and can't wait to see if it helps even more.