Cooling towel or your neck of forehead. If you have a cooler with you, you can cool the towel back down in the cooler periodically.
Stay in the shade whenever you can, you can bring a parasol if you have your hands free to carry one.
Avoid being out during the hottest part of the day if possible. Usually, 1:00–4:00 more or less in North American in the summer months.
Loose clothing.