Install a low-flow shower head, which restricts the water ouput to no more than 2.5 gallons per minute. The shower heads are inexpensive and screw into existing fittings.
Aerators screw into the faucet threading and cut the water flow from 3 to 4 gallons per minute (the rate on older fixtures) to as little as a half-gallon.
When brushing your teeth, turn the water off and save up to 2 gallons of water in two minutes of brushing.