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.
When packing, bring your own travel size toiletries, rather than using the soaps, shampoos, and lotions provided by the hotel. When the bottles are empty, refill them from a larger container rather than buying new travel sized products.
And be sure to take advantage of hotel programs that allow the cleaning staff to skip changing your sheets and towels daily! If your hotel doesn't have this program, leave a note on your bed asking to keep your same sheets and towels.
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.
Speaking of photos- don't use a disposable camera (obviously) and consider a digital camera over a film camera. You'll only print the shots that you want to keep, and won't need to buy any more rolls of film!
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).
Also when eating- skip the fast food restaurants where your meal is served wrapped in paper, with disposable silverware and paper cups- either pack supplies to make your own meal (because picnics are great fun anyway!), or eat somewhere that uses real dishes.
If you fly, look into carbon offsets.
Think about taking a train.
Or a road trip with some of your friends- take your carpooling to the next level! This also means splitting gas, which saves money...