This information is from WWF; you can learn more from visiting their site.

• Look at eco-tourism options. Many travel companies now offer holidays which support sustainable development, do not impact negatively on the environment and provide financial support to local communities.
• Respect the environment and stay on trails and public footpaths. Do not remove plants or feed animals, and never litter.
• Conserve water and take showers rather than baths. Use a refillable water container, sterilizing water when necessary rather than buying bottled water.
• Limit energy use, including your use of air-conditioning and hot water. Turn off all lights and taps when you leave hotel rooms.
• Minimize your use of personal care products and detergents to wash linen, and reuse your hotel towels and bed linen.
• Dispose of sanitary waste properly. Don’t flush cotton buds, condoms, tampons and plastics down the toilet, or you might just find them on the beach!
• Recycle newspapers, magazines and your beverage containers (many can be returned for refunds), and reduce the number of bags, napkins and disposable cups you use when you eat fast food.
• Rent a bike. In many places you can rent them out. They are not only environmentally friendly and cheap, but also a great option for sightseeing.
• Take a hike. walking in cities or going on a hike in the countryside is great way to avoid emissions and yet discover some of special things about where you are, things you may otherwise simply "drive by".
• Reduce the impact of your recreational activities by avoiding sports which have a significant harmful impact on the environment, or choose more progressive establishments (e.g. golf courses which recycle water).