Ask questions. Consider asking your travel providers (tour operators and hotels) the following questions that are suggested by Lonely Planet:
How are they dealing with the main environmental issues facing them?
Do they employ local guides, leaders and staff and provide training opportunities?
Do they limit the size of their groups to minimize environmental and social impact?
Do they have a 'green' purchasing policy?
Do they work with the local community? If so, what proportion of their revenue is redirected to that community?
What information do they offer their clients on responsible travel?
Look for a green hotel. When looking for lodging, try to find hotels/motels that use compact fluorescent lights, use nontoxic cleaning products, and have a towel and linens reuse program and a recycling program for guests. Check out the Green Hotel Association for more ideas.
If you plan to go to several places, plan your trip in advance and avoid backtracking. That way you'll minimize your travel time (saving energy used to transport you and giving you more time to vacation).
Take nothing but photos. Leave nothing but footprints. Kill nothing but time.
Go local. Eat local foods, hire a local guide, buy local crafts for souvenirs (but avoid things made from endangered species, hard wood, or ancient artifacts).
Check out Global Exchange for opportunities to volunteer and get hands-on experiences in other lands.
Check out Wildland Adventures. In 2005, Wildland Adventures was given Conde Nast Traveler’s Green List award for its “commitment to preserving natural surroundings and contributing to local cultures.”
Remember that not everything that claims to be "eco-tourism" actually is.