An excerpt from The Green Hotels Association

When packing for a trip, bring (1) a handful of safety pins and different colored small beads, or different colored safety pins; (2) one or two plug-in night lights; (3) a reusable mug; and (4) a permanent marker. You'll see why below.
Ask the staff not to change your sheets and towels every day. To keep track of which towel belongs to which family member, attach a safety pin and bead to each towel and assign a different color for each family member. Remove the pins before you check out.
Whenever you leave your room, turn off the lights, radio, TV, and heating/cooling. Close the drapes.
Plug in a night light instead of leaving a room light on all night.
Participate in any recycling efforts the hotel promotes.
Keep the soap wrappers and bring home used bars of soap.
If the hotel provides complimentary newspapers, once you are done with them pass them along to someone else or ask management whether they can be recycled.
Leave unopened amenities in the room and take home opened ones.
Use a permanent marker to write each person's name on the cups in the hotel bathroom.
Don't let the water run while brushing your teeth; take short showers.
Turn off exercise machines, saunas, and other equipment when you are done.
Take only the brochures and tourist information you need and return the rest to the display racks.
If the hotel has a free breakfast or happy hour, take only the food you will eat. If you have seconds, use the same utensils. If the hotel uses disposable cups, bring your own reusable cup.
If you don't drink water with your meal, ask that the wait staff not pour you a glass.
If the hotel already embraces green-hotel practices, thank the managers and employees and tell them you appreciate it.