The worst for me is when the sun is highest, between 12:30 and 3 pm, give or take. If I can't avoid riding during that time, I'll stop as often as necessary wherever I can find shade. Ideally I'll go into a convenience store or some other air conditioned place, because that's best for bringing your temperature down quickly. Also cold water on my lower back or a small bag of ice in my jersey pocket works wonders.