Dr. Nicholas Perricone says that caffeine will actually make you gain weight because of the way it interferes with your sugar metabolism. I don't really have a strong opinion about him one way or the other, and I didn't lose any weight when I quit coffee (I still drink green tea, but I figure my caffeine intake is about 20% of what it used to be), but there it is FWIW.
ETA: I re-read your OP and see that you're drinking tea, not coffee. There are several of the catechins in green tea that are touted as beneficial for weight loss - not the caffeine.
Speed comes from what you put behind you. - Judi Ketteler