Drink a lot of water during the day? Then ditch the plastic water bottles. Instead pick up a nifty stainless steel reusable water bottle. Most will keep both warm AND cold liquids, but I love it for plain old water.
Prices can be anywhere between $10-$25. Totally worth it. Both for the world and for your pocketbook.
Great suggestion on the SIGG bottles, and you may also like this list of more green ideas: http://balancephoto.blogspot.com/2009/04/green-er.html
Please do not use SIGG, they have questionable lining, use something like Kleen Kanteen or Earthlust. Much better for you!