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 water sparingly: Many communities face water shortages and water usage costs money. One small gesture you can make is to take quick showers.
Carry a Reusable Water Bottle: If tap water is safe where you are traveling, reduce your environmental impact by avoiding bottled water and bring a reusable water bottle with you.
Save electricity: Turn off lights, air conditioners, and heaters when you are not in the room.
Reduce, Reuse, Recycle: Always try to use less - reduce. Recycle anything as much as you can. If not, think of new uses for products before you throw them away.
Use local and public transport whenever possible
Buy Local, Eat Local
Don't litter!: Try to find a container to dispose of your litter.
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.
Research your destination: Learn about its history, political situation, current events, cultural groups and intercultural dynamics, religion, geography, cuisine, transportation, etc.
Find out about local taboos and customs by asking people who have traveled before you and by consulting guidebooks, and then respect them.
Dress appropriately: Respecting the dress code where you are is very important, especially around religious sites.
Be photo 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.
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.
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.
Contribute something to the place or community you are visiting, beyond just the money you are spending to get what you want: 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.