Increase your intake of non-alcoholic, non-carbonated, caffeine free beverages, such as water and juice.

Wear clothing that is light in color and loose fitting.

Avoid the outdoors during extreme heat, stay out of the sun.

Stay in an air-conditioned environment if possible.  Shopping malls offer relief if your home is not air-conditioned.

Check on the elderly, they are especially susceptible to heat related illness.

Eliminate strenuous activity such as running, biking and lawn care work when it heats up.

Eat less food that increases metabolic activity/heat.  Proteins are an example.  Increased metabolic heat increases water loss.