Starting with something easy and basic:
Buy a re-usable bottle. Sigg seems to be the bottle of choice since the Nalgene BPA estrogen scare. (Nalgene is in the process of reissuing their bottles BPA free, these are currently available at their website.)
But for now Sigg is much easier to find (not to mention much cuter). They're available at Whole Foods, EMS, and REI as well as seemingly every health related or health conscious store I have wandered into recently.

If you already have a bottle, start thinking about where else you might use it... (For instance, I recently noticed that, while I would bring mine along in situations where I might otherwise need to buy a bottle of water, I was still using the paper cups on the side of the water jug at work while my Sigg sat empty in my bag.)