Spotting a good quality green tea

Aroma: Good quality green tea has an amazing aroma. It is very fresh and has a very distinctive, almost sweet smell. Generally speaking the stronger and more distinctive this smell, the more flavoursome the tea is likely to be. It is a very good indicator of the quality of the tea.
Taste the dry leaf: Old leaves or poor quality leaves will have very little flavour at all, dull and almost tasteless in some cases. A good quality green tea leaf will expand into a multitude of flavours in your mouth, making it a good indicator of the quality of the green tea you will be able to brew from it.
Use by date: Green tea should be regarded as a fresh produce. Although green tea will not rot, or go bad for a very long time, it still has a relatively short 'fresh' period during which its flavour will be most distinctive. In fact if stored in poor conditions, Japanese green tea will lose much of its flavour relatively quickly. Generally speaking, enless stored in special conditions high quality green tea will last for about a year. Beware of Japanese green teas that have a relatively long shelf life, i.e. 2-3 years.
Brewing: Never use boiling water on Japanese green tea or over brew the leaves. Both these things will make your tea very bitter. Boiled water should be cooled to 70 to 80 degrees Celcius, and the leaves should not be brewed for longer than 30 seconds to 1 minute. If a Japanese green tea needs boiling water or 2 to 3 minutes brewing time then the chances are the quality of the tea is not that great.
Storing Japanese green tea

There are 3 main enemies of green tea, heat, light and air. All three will contribute to the rapid deterioration of your tea. Tea will also absorb odours, so if not stored in an airtight container, beware of placing the tea near other strong smelling foods. Generally speaking, keeping the tea in a non transparent container that is airtight and not in direct sunlight is a good place to start.