With a permanent tea filter like one of these and one of Contigo's wonderful, absolutely-will-not-leak insulating mugs, you can brew your loose-leaf green tea and take it on the road with you.
First, you start out with the tea, filter, and cup, like so:
Next, you place tea in the filter, and put the filter inside the cup.
Pour in hot water — but make sure the water is not too hot — and let the tea steep for about 3 minutes. (For second or third steepings, you only need to wait a minute or so.)
Now remove the filter, screw on the mug's top, and you've got a hot, leak-proof cup of green tea ready to take anywhere.