Grow your own food, or at least some of it. Even if you live in an urban area you can grow some big pots of veggies on your porch, deck, roof, or windowsill. Lessen your time in front of the TV/comp, spend more time outside, turn off the lights when you can, bike/walk to work/school/other destinations, use cloth shopping bags and shop for local foods and products. This is a pretty good site, they have lots of info about homesteading/farming/green living/sustainability: http://www.motherearthnews.com
Good luck!!