Before you leave home
Turn your water heater down to "Vacation," or the lowest setting.
Turn off your AC / heat or adjust the thermostat to protect plants.
Appliances, such as TVs and cable converter boxes, should be unplugged, because they can draw or "leak" as much as 40 watts per hour each, even when they are off.
Carry bottled water with you. Refill as needed. Pack a permanent marker for writing names on bottles and lots of other things.
Reduce the amount of water used for bathing or showering.
Turn icemaker off (lift the metal wire) to prevent flooding should it break while you're away.
Lower the temperature of waterbed heaters at least ten degrees.
Stop your newspaper. You may be able to donate it to a school.
During Travel
Enjoy walking tours.
Walk where sensible.
Avoid styrofoam. Carry your own cup.
Use the bus or local transit system.
Take only the brochures or maps necessary at welcome centers. Return others to the brochure stand. (And who needs those maps with Travel Itineraries?)
Don't allow anything to be thrown out of your car windows -- ever.
Take photos with a digital camera. Disposable cameras are very wasteful and expensive.
Pick up at least one piece of litter every day - especially at national tourist locations, which are vulnerable to huge tourist populations.