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 visitor.
If you go camping, make sure you have any necessary permits and follow local park rules.
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.
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.
Chose destinations based on their demonstrated commitment to sustainable practices including their human rights record, environmental conservation record, commitment to peace, etc.
Don’t pick up and take home natural resources such as shells, plants, animal bones, etc.
Stay in locally owned accommodations, eat at locally owned restaurants, and hire local guides.
Minimize your impact on the environment.
Engage with locals and participate in the local community.
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.
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.
If it is within your budget, contribute money to an organization involved in carbon offsetting every time you fly. They will, in turn, contribute money to worthy organizations that are involved in projects that seek alternative energy sources, plant trees, etc. in order to reduce the amount of ozone-depleting carbon in the atmosphere, largely caused by air traffic.