Don't cook wastefully!
It's saddening how often people engage in wasteful habits when it comes to food. So much money and resources could be saved with more careful habits.
Eat your veggies uncooked. Even frozen vegetables, when thawed, can make delicious salads.
When you do cook vegetables, cook them lightly.
Put a lid on it. Covered food takes less energy to cook - and less time. It also doesn't splatter, either.
Use the smallest pan possible. The bigger the pan, the more metal will have to be heated to reach cooking temperature, and that means more energy will have to be used. As a corollary, don't put a pan on a burner that is too big for it. (IE, the burner peeks out from under the pan.)
Unless you're baking, don't preheat. Roasted foods do not require a preheated oven.
Get reusable containers to hold lunch, leftovers, and things you might put in the freezer.
Consider freezing leftovers. They can be thawed later when food is needed quickly.
Don't throw edible bits out with the scraps. One fellow I know threw away almost 1/3rd of the peppers he cut up for dinner! Some people like to get artistic with their food, cutting it into cute shapes and arranging it to look nice. However, perfectly edible food cut away during preparation may be thrown in the trash. When prepare your food, do it in such a way that all of the edible parts are used. Maybe save the edible scraps for a snack later.
Some people put more food on their plates than they can eat. Once they have eaten their fill, they simply throw the rest away. To avoid this disgusting and wasteful habit, take less food than you think you can eat, or put the leftovers in the refrigerator.
Don't peel foods such as kiwis, apples, cucumbers, and potatoes. The skins contain the most fiber.
Eat fruits and vegetables directly instead of making them into juice. You'll get more fiber.
Get proper cookware - the kind that will last you for years.