• Learn about current environmental issues in the places you are visiting. Different regions will have different situations based on their ecosystems. Learn about the effects of mass tourism on beaches, mountains, wetlands, deserts, etc. and then seek to counter those effects.
• Use accommodations that have a reputation for being sustainable (they recycle, use alternative forms of energy, are owned by or employ locals, contribute to local causes). Increasingly, there are regional and national certification systems that accommodations can obtain if they are sustainably operated, much like the organic labeling system. Check to see if there are any local certification labels that can help you to determine where to stay. Search the Internet to do this (country name + tourism certification) or inquire with the visitors bureau or local tourism offices.
• Use water sparingly. Many communities face water shortages and water usage costs money. Take quick showers.
• Save electricity. Turn off lights, air conditioners, and heaters when you are not in the room.
• Don’t litter! Even if you notice the locals doing so, try to find a container to dispose of your litter. Always recycle if possible.
• Don’t buy products made from endangered species or valuable, historical, or cultural artifacts. Ask about where a product comes from. Many of these products are illegal to export. Report incidences to local or national conservation organizations.
• Don’t disturb the wildlife. Maintain a proper distance at all times. Don’t use loud, motorized equipment among small communities of people or in areas where there is wildlife.
• Don’t pick up and take home natural resources such as shells, plants, animal bones, etc.
• If you go camping, make sure you have any 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—airplanes produce massive amounts of ozone-depleting carbon dioxide.
• Buy locally produced products and services. Don’t bargain too much over an extra dollar or two that will go a lot farther for your seller than for you.
• Go Local. Stay in locally owned accommodations, eat at locally owned restaurants, and hire local guides. Usually, smaller equals better. If you decide to go on a guided tour through a tour agency, ask about their sustainability practices (e.g. what do they do with garbage generated, who do they employ, who is the agency owned by?)
• Contribute something to the place or community you are visiting, beyond just the money you are spending to get what you want. Donate some money to a good and relevant cause either before, during, or after your visit. Plan ahead to contribute some time, and volunteer at an organization that you deem worthy. It would be wise to research what organizations exist and contact them to inquire whether they receive volunteers before you leave.
• Choose destinations based on their demonstrated commitment to sustainable practices including their human rights record, environmental conservation record, commitment to peace, etc. Check out Ethical Traveler  for more information.
• Research your destination. Learn about its history, political situation, current events, cultural groups and intercultural dynamics, religion, geography, cuisine, transportation, etc.
• Learn at least a few basic phrases in your host community’s language. Learn how people greet each other and practice that greeting. Body language is also important. Be astute and adapt your body language appropriately.
• Find out about local taboos and customs by asking people who have traveled before you, by consulting guidebooks etc., and then respect them.
• Dress appropriately. Respecting the dress code where you are is very important, especially around religious sites.
• Be snapshot savvy. Don’t experience your entire trip through the lens of a camera. Ask locals before taking photographs of them or activities they are involved in.
• Learn about something you’re interested in while you travel. Do you have a passion or hobby? Find out how people in another culture approach or deal with the same theme.
• Get off the beaten path. Look for events going on that are not mentioned in guidebooks and seek places that are not overcrowded with like-minded tourists. Go where the locals go; however, use your discretion and don’t infringe on people’s private activities and spaces.
• Bring small, thoughtful gifts from home if you know that you are going to be spending time with a local family or in a community.
• Beggars. In many cities in the world you will encounter both children and adults begging. Generally speaking, giving money to children is not a good idea. Depending on you where you are, the implications for giving to beggars are different. Search the Internet and local travel guides for local rules and recommendations.
Sustainable Travel Tips
• Be Safe! Never compromise your safety. Be aware and use good common sense. Whenever possible, travel with someone else.
• Be flexible, patient, open-minded, and light-hearted. Learn to see the humor in your mistakes and in moments when you feel frustrated.
• Be mindful of others by keeping your voice down and practicing your listening skills. Learn to be quiet.
• Ask for help and you’ll probably get it.
• When you return home, donate money, volunteer, or get a job working with worthwhile causes in the regions you have visited or with the issues you have encountered.
International Center for Responsible Tourism: http://www.icrtourism.org/ 
Tree Flights: Sponsor some trees to absorb your carbon emissions at http://www.treeflights.com/ 
Association for the Advancement of Sustainability in Higher Education: http://www.aashe.org/