Re: green tea
Green tea is the healthiest of the caffeinated teas, and it has much less caffeine than black tea or coffee. If you're a Kapha dosha, then you actually could use a bit of an "upper," really.
But if you find yourself getting headaches on your weekends when you sleep in, or suffer from insomnia or the 4:00 p.m. crash, that is a major sign that you're doing your body a disservice.
And as with any drug, you should take breaks from it. So periodically just don't drink it, for a week or two.
I must admit I'm fond of Sobe's Green Tea drink and have it once a week or thereabouts, and I drink Yogi Tea's Green Tea Chai occasionally, as green tea is SO beneficial for the kidneys.
Caffeine has been linked to certain issues such as
Breast Cancer etc., but just keep it to a moderation and you should be fine. :)