If the problem really is simply overheating, and not bonking/dehydration/electrolyte deficiency, wearing something long sleeved and white will help, especially cotton. Cotton is helpful in this case for the same reason it's terrible most other times: it holds moisture against your skin where it cools you as it evaporates. The cheapest, thinnest long-sleeved white cotton t-shirt you can find is a good start. The higher quality ones have thicker fabric, which may not let as much air through, but the super-cheap ones are often very thin and light. You can also fill a tube sock with ice, tie the end closed, and drape it around your neck. And of course you have to kep your pace down to where you can manage the amount of heat you're generating.
And if you're really overheating, you might just have to live with frequent stops to eat popsicles in the shade.
Also, avoid sunburn, because sunburned skin doesn't sweat as effectively and therefore doesn't dissipate heat as well.