Before air-conditioning, people cooled off in summer in part by adjusting their personal routines. Some of their strategies are still useful.
Watch What You Consume
Drink plenty of liquids, but limit caffeine and alcohol. Enjoy spicy foods and even hot herbal tea; they make you perspire, which cools you down.
Wear Loose Clothing
Choose cotton, linen, or similar breathable fibers, and avoid anything synthetic.
Pare Down Furnishings
Roll up carpets, and store heavy drapes for the summer. These items hold moisture, making a space hotter and air conditioners, if you use them, work harder.
Avoid Chilly Places
If you don't have air-conditioning at home, that is. Abrupt changes in temperature will only make the heat seem hotter. Why not just find a shady spot with a nice breeze, and relax for a while?