Recently, I had the good fortune to attend a conference on study abroad, at which the presentation "It's Easy Being Green: Advising Tips for Earth-Friendly Study Abroad" was given. While not all of the information imparted applies to vacationers, travelers and volunteers, much of it does. I have taken those parts of the presentation that can apply across the board for international travel and would like to share it with you here. Thank you to the presenters who took the time to put this information together: Jessica Haas, Katherine Yngve, and Airika Coblentz.
Simplicity is okay.
Start where you can.
Don’t be intimidated.
Learn about current environmental issues in the places you are visiting.
Use sustainable accommodations (they recycle, use alternative forms of energy, are owned by or employ locals, contribute to local causes).
Use water sparingly. Take quick showers.
Save electricity. Turn off lights, air conditioners, and heaters, unplug appliances to prevent “vampire” energy use.
Don’t litter!  Always recycle if possible.
Don’t buy endangered species products or valuable, historical, or cultural artifacts.
Don’t disturb the wildlife. Maintain a proper distance at all times.
Don’t take home natural resources such as shells, plants, animal bones, etc.
If you go camping, make sure you have necessary permits and follow local park rules. Pack out what you pack in. Stay on trails.
Choose your recreational activities wisely. Low impact sports that don’t involve a lot of equipment or fossil fuels and that don’t disturb the environment or local communities are preferable.
Use local and public transport whenever possible. Take a train or bus.
Bike or walk. Try to fly less. 
Carbon Offsetting. If it is within your budget, contribute money to an organization involved in carbon offsetting every time you fly.
Pack light. Bringing fewer, higher-quality items that will last your entire [trip], and leaving behind toiletries that you can purchase while abroad can save on fuel and hassle for you.
Change your laundry routine, do full loads, use cold water, use concentrated, plant-based detergents,  and air dry your clothes.
Encourage others, invite your [travel buddies] to visit the local market with you for this week’s snacks or get together with your local friends for a group cooking night.
Minimize using AC by opening a window, turning on a fan, taking a cold shower before bed, or [reading] outside.
Buy used or ask about free hand me downs. 
Eliminate excess mail.  Cancel or place a hold on newspaper or magazine subscriptions.  Call catalog and junk mail senders and have your name removed from lists.
Use the airport lavatory, not the one on the plane.
And here are some interesting facts pulled from treehugger.com:
34 million tons: Amount of carbon dioxide emissions that would be saved if every U.S. household used only cold water for washing clothes--that's nearly 8 percent of the Kyoto target for the U.S.
700 pounds: Amount of carbon dioxide emissions saved each year by line-drying your family's laundry. You'd save 75 bucks, too.
99 pounds: Amount of carbon dioxide emissions saved per household each year by running only full loads of laundry.

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