Understand
Protecting this planet we live on is everybody's responsibility. Future travellers should be able to enjoy it as much as today's travellers.
Prepare
Turn off furnaces and water heaters. (''If there is a risk of pipes freezing, turn the water heater to it's lowest setting rather than off'')
Turn off the mains water supply. This will ensure no no flooding in the event of a water pipe failure while no one is home
Switch off all lights
Unplug TVs, video recorders and other devices that continue to draw power in their standby mode.
Get around
Walk, use public transport, or ridesharing schemes where possible rather than renting a car. Use air transport only for long distance travel where there is no rail transport available.
See
Don't use disposable camera's, they are wasteful and generally of low quality. Read Travel photography for some tips on selecting a camera that will fit your needs.
Do
Leave all areas you visit clean. If no bins are provided, take your litter with you and dispose of it responsibly. This includes cigarette butts.
Eat
Drink
Sleep
On leaving a hotel room, switch all lights off
Find hotels and resorts that encourage eco friendly practices [http://www.ecotrotters.com]
Flora and fauna
Don't kill anything that is not a common pest. Note that not all insects are pests, some insect species are even protected and you may face is stiff fine if you kill them.
Clean you hiking boots before moving from on area to another. Seeds often get stuck to the soles of boots or attached themselves to the upper portions or laces. These seeds can easily become transported to an area where they do not naturally occur and become an invasive species there. Rinsing the soles of your boots also reduces the risk of spreading disease.
Fire poses a serious risk to many areas around the world. Though fires do occur naturally and are even required in some ecosystems, human activities can increase the rate at which they occur and cause substantial ecological damage. When making a campfire, ensure that it is in an area clear of vegetation and do not leave it unattended while burning.
Buy
Many species are under thread because they are killed specifically for the tourist trade. Starfish in coastal regions is one such specie. Do not buy any animal products where the animal have been killed simply to serve as a tourist memento.
Back home
Do not print all your photos, especially if they are digital. Select the best ones and only print those.