1. Buy a Popsicle, ice cream, or some other frozen yummy treat.
2. Drink plenty of cool water. During summer, you sweat more, and you have to replenish the water in your body. So drink a nice cold glass of water, and keep a bottle with you always. A tip to stay more cool is fill up a water bottle and put in the freezer for 30 min to an hour, and freeze another to take outdoors and the "iced-water" will last much longer. Don't risk dehydration.
3. Find a friend, and go for a swim, have a water fight, or just dump buckets of water on each other. A weird but fun way is bring a nice cool bucket of water and pour it on your trampoline if you own one and then you created your own slip and slide.
4. Place an ice pack on your wrists. It cools the blood in your veins, and instantly makes you feel cooler. You could also run your pulse under some cold water or just splash some cold water on your face to stay at a desirable temperature
5. Sleep with a sheet when you go to bed, and have a fan directed toward you. You can also dampen your sheets in water and the air blowing from the fan will make you feel refreshed, until it all evaporates.
6. Turn off unnecessary lights, TV, oven, etc. During the day, keep off the majority of your lights, since they produce energy and heat!
7. Shower or bathe frequently in mildly warm, or cold water to stay refreshed.
8. If you have an air conditioner, turn it on. Keep the air filter clean to avoid clogging or fungus may grow there. For best results, close the doors to the rooms that have the air conditioning on. This will trap the cool air in the room, keeping you cooled down.
9. Make sure the setting on the AC is on "high" to move the air through faster and farther. This is not the "colder-to-warmer" settings. That uses very little additional power, but may make a louder "shushing" air sound.
10. Use a portable fan set at the desired setting to keep you at your own comfort zone, too.