Once you have your low-flow faucet aerator installed, your behavior can also help keep water flow down. Be sure to turn off the water while you're brushing your teeth—some dentists even recommend a dry toothbrush. You'll save 6 gallons of water each day, assuming you're diligent about brushing twice a day.
Boys: If you shave with a wet razor, put a stopper in the sink and don't leave the water running. Half a sink-full of water will do the job.