If you're worried about sitting all day, drink more. Water, tea, coffee, anything really. You'll have to go to the bathroom more, that WILL get you out of your chair.
I'll let everyone make their own determinations about milk -- I would be a skim-only person, and then only sparingly. But if asked for a recommendation, I would say limit milk intake to much less than water.
Sugarfree soda = the devil.
Sugarfree juice -- the real stuff -- is probably ok in moderation. However, a good chunk of the health value in fruits comes with the pulps and skins and pithes and the like. Fruit juice is really just a portion of the healthy nutrients one can get from fruit. Moderation is the key, as the glycemic load of fruit juice, even just the squeezed stuff, can really be high.
Never mind what all this stuff does to one's dental health, which is harsh (even milk is a culprit.) I simply can't encourage those of us who sit at desks in this job to drink enough water. 64 oz is the recommended minimum a day, but I think it should be higher.
Caveat: this works for me, so your mileage may vary.
Green Tea keeps me moving.