Drink plenty of cool fluids, even if you are not physically active and even if you are not thirsty. If you are physically active, drink 2-4 glasses (16-32 ounces) every hour.
Wear lightweight, light-colored, loose-fitting clothing. Add a wide brimmed hat, sunglasses and sunscreen (SPF 15 or higher).
Limit physical activity, stay indoors in an air-conditioned space or, when available, buildings open to the public as special, "Cooling Centers." Electric fans will not prevent heat-related illness, and may actually be harmful, when the temperature climbs into the high 90s.
If working outside in the heat, monitor coworkers and ask them to do the same for you. Check on the elderly at least twice a day. Check on infants and young children frequently. Check on those who are overweight, ill or who are on medication.
Use common sense. Avoid hot foods or drinks and heavy meals. Make sure animals and pets have plenty of fresh water and shade. Consider bringing pets inside and consider wetting down outside animals.
Do NOT leave any person or pet in a parked car for any length of time for any reason.