Search Environmentally Friendly Hotels to find hotels where you are going.
Ceres has a Green Hotel Initiative and for a very easy step, use their printable guide to leave with hotels you visit. If green options aren't offered at your hotel, let staff know they would be appreciated. Make sure to thank hotel staff for everything they do right!
Instead, bring your own or buy in bulk to cut down on wasteful packaging.
Airbed and Breakfast is an online marketplace for booking rooms (or mere couches and airbeds) with locals as an alternative to hotels, or look for a hostel.
Many communities provide non-commercial lodging. These are not merely cheap alternatives to hotels and shouldn't be treated as such: often they are real communities, asking less of your money and more of you -- hopefully including your reciprocation -- and providing more than a room. Often they are focused on cultural exchanges and friendships. Explore:
couchsurfing.org -- the most famous of the web 2.0 couch surfing sites, bewelcome.org, belodged.com, hospitalityclub.org, globalfreeloaders.com, place2stay.net, or Servas international cultural exchange.