If you're a dedicated carnivore like myself, it's a difficult prospect to give up eating hamburger, steak, pork chops, etc. Trust me, speaking as a one-time vegetarian who caved one summer at the smell of a steak on the barbecue, I know. But the fact remains that probably nothing a person can do on an individual level has as much of a positive impact on the environment as cutting back on the amount of meat they consume.
Meat production puts an incredible strain on the world's land resources -- using up to 30% of the Earth's entire land mass either directly or indirectly -- and actually has more impact in terms of the amount of greenhouse gases it pumps into the air than even petroleum-fueled transportation. When major agri-business is clearing the rainforests (which is itself enormously damaging for the environment) they're doing it largely to make room for meat production. In terms of water use, it takes 2500 gallons of water to produce one pound of meat. And then of course there are the ethical issues -- not to mention dietary and quality considerations -- surrounding the conditions under which most livestock is raised and killed in order to provide the meat consumed worldwide.