The Mr. Clean Magic Eraser cleaning pad is absolutely "magic" at removing tea stains. My wife will only let me use one cup for tea because of the stains and my ingenuiTea was getting so cloudy I was considering replacing it.
One quick swipe with the Magic Eraser and the coffee cup is better than new. It took only slightly longer to restore the ingenuiTea to crystal clear. I just wish it came in a toothbrush size so I could clean the little crevices.
I am considering trying it on my teeth. ;-)