Get out of the sun, drink plenty of water. Ideally you should have plenty of water atleast half an hour before you go out into the sun.
Eat more fruits and vegetables. This is because they have fairly high water content and good salt balance. Drink diluted electrolytes.
People who sweat heavily tend to loose a lot of potassium and sodium. There are many over the counter electrolyte packs.
Avoid salt tablets, alcohol, caffeine, smoking and do not bare your chest.
Do not suddenly go out into the sun from an air-conditioned room or do the opposite. Learn to acclimatise slowly.
Dousing your head and neck with cold water will help if it is hot and dry.
Use a newspaper, handkerchief or a traditional hand held fan to keep yourself in cool breeze.
Wear light and light coloured clothes. the preferred coloured is white because it reflects the heat. Use cotton or polyester blends because such cloth material can breathe.
Use an umbrella or a hat, preferably one that also shades the neck and is well ventilated.
Source - health tips