Toilets typically use 1.5 to 3+ gallons of water per flush. Unless the toilet uses a special gray water system, it uses the same tap water as your sinks and showers.
To save some of this water, place your water bottle in your toilet tank. This will keep the bottle out of the landfill and save water every time you flush! This method reduces the amount of water your tank uses by taking up some of the volume. This is an idea adapted from the '70s, when folks placed bricks in their tanks, which is not recommended as it can damage the plumbing (http://consumerist.com/consumer/home/should-i-put-a-brick-in-my-toilet-to-reduce-water-use-267872.php)
I used pictures from this instructable: http://www.instructables.com/id/Conserve_Water_Convert_your_existing_toilet_into_/
as my camera is out of commission