tips for great green tea:
1) buy the good shit. republic of tea makes some great green teas.
2) don't boil the water, heat it until you see steam wisping from the pot.
3) don't squeeze the bag as it can release sediment or other bitter-tasting parts of the tea leaves.
and to answer your question desman - you can steep the tea bag for as long as you want, it really just depends on how strong of a taste you like. if you are using flavored green tea (especially lipton), it's best to not leave the bag in the cup for long because all of the chemicals used for flavoring will make it bitter.