There's more to being a good citizen of Planet Earth than reducing, reusing and recycling (although that's also a great start!)
Also consider not eating meat, or better yet, not consuming any animal products! It's not just better for our furry (and feathery!) friends, it also reduces your carbon foot print!
Use public transportation or bicycle when you can, and try to avoid airplane travel (a huge source of carbon emissions!) Keep your home cool in the winter and warm in the summer - every degree closer to the ambient temperature saves that much energy!
Also, be an advocate for our planet! Write to your representatives so that they know you support sustainable actions! Volunteer for an environmentalist cause, whether it be picking up trash at your local beach or raising money for the Sierra Club. You can make a difference!