Dress in light weight, light colored clothes. Drink plenty of liquids - don't dry out. If you don't have an air conditioner, don't hesitate to open windows and use a fan.
Older people and persons with weak hearts should particularly avoid exertion during extremely hot weather.
Stay wet and take long and frequent baths.
Seek medical help immediately if you begin to suffer symptoms of heat exhaustion. Such symptoms include vertigo (dizziness), nausea, weakness, heat cramps, headaches, and profuse sweating.
This is a good time to drink Gatorade or other drinks with electrolytes in them.