Eating healthily is as important for the envionment as it is for your body.
The one biggest thing you can do is buy as little packaging as possible. When you're buying fruit and veg, you don't need all those individual plastic bags, for example.
We all know that too much meat isn't good for your health, and eating too much of it isn't good for the environment either: it takes 100 000 liters of water to produce 1kg of beef, compared to 900 liters for a kilo of wheat, and it produces 12 times the same amount of greenhouse gas as a kilo of chicken.**
Don't buy exotic fruit in the middle of winter: it takes tons of kerosene to transport them by plane, which means tons of harmful gases released into the atmosphere.
When you're doing the shopping, buy big, sturdy, re-usable bags and take them with you every week instead of using 20 flimsy carrier bags - it's far easier to transport your shopping in proper bags (they don't break!) and you don't throw them away every time.
* British Commission Report on Durable Development