Hydration, hydration, hydration. Easier said than done.
For some reason, I find it difficult to drink large quantities of plain water on a constant basis. After all, it's just so... plain.
So rather than going with the traditional office drinks of coffee or soda, or stocking up on 'sports drinks', I've opted to just toss a teabag into the cold water bottle.
It keeps the water as untainted as possible, while adding just enough flavor and variety to make constant hydration a pleasant activity. I happen to like green+jasmine tea, which is almost flavorless in cold water, but mint, orange pekoe, even earl gray are excellent. If you want the tea stronger, just it in the sunlight for a bit.
My latest addition has been 2 lemon slices per liter, which adds a huge amount of flavor. By slicing the lemon, and then freezing the slices in a tupperware container, I can keep them indefinately in the office fridge, avoid the muss and fuss of slicing each time, and I even get to add a nice chill to the drink.
If you try this, you might consider using freezer wax paper to keep the slices from freezing to each other. Also make sure that whatever size you cut the slices to, that they'll fit into your bottle once they're frozen stiff.