Traditionalists say that green tea tastes best when steeped for less than two minutes in water that hasn't quite reached the boiling point (about 180 degrees). Others say that using water at boiling temperature (212 degrees) and steeping the tea for at least four to five minutes increases concentration of the antioxidant (polyphenol) in the tea. Maybe it's more healthful to brew the tea longer, but I prefer the more delicate taste of a briefer steeping time.
However you decide to make it, loose-leaf teas are, of course, preferred. A teaspoon of leaves is enough for an eight-ounce cup.
-- Cecilia Hae-Jin Lee