Rinse out a sturdy thermos with water, and then put it in the freezer the night before, so that the inside liner gets nice and frosty (like a frosted beer mug). Just before leaving the house, pour freshly made juice into the thermos right up to the lip - it's important not to leave an air pocket. Screw the top on the thermos holding it over the sink, since the full jug will probably overflow a little. If you leave any air in, it will warm up, and the juice will lose some of it's potency. If possible, store the thermos in the refrigerator at the office. Drink the juice all at once. Don't drink a little and recap it. You might consider buying 2 small thermoses, if you plan on drinking at two different times. After about 24 hours of storage, it may become toxic (and will turn brown). So don't keep it over a day. Melon and cabbage juice don't keep well, so maybe you shouldn't take them to work.