How can a person with limited income become a more responsible consumer?

Let’s face it – it’s easy to be responsible when you have the money to do it. “Buy organic and local” is great for food if you want to spend 2–3 times as much sometimes – but if you can’t, that’s a problem. And if you can’t really afford much in the way of clothes, furniture, products, etc., K-Mart, Wal-Mart and the other Mart form superstores become it because you need them to use their economies of scale and integration techniques to subsidize your purchases.

What are affordable ways that I can choose to buy items and food that cause less harm to the environment, international human rights, employee rights, etc.? What are the cheapest resources? Are there ways that purchases can be subsidized? And where are these resources national?