Leaving the refrigerator door open lets the cold air out, meaning your fridge has to work harder to maintain its inside temperature.  Also, try to remember that oven-baked foods will cook more quickly and evenly if you can avoid opening the oven door repeatedly to check on the food.  Try to pull everything you need from the refrigerator in one go. Use an oven timer for help with preheating, checking on your food, and remembering to turn the oven off. Putting frozen items in the refrigerator to thaw is safer than thawing them on the counter. Frozen foods also help to keep the interior of your fridge nice and chilly as the cold seeps into the surrounding air. Thawing foods like meat or casseroles all the way will reduce their cooking time once you're ready to start. If you're only using a small saucepan to cook with, put it on the smallest burner. Using a larger burner will allow heat to escape into the room, and does not cook your food any faster. This can be especially helpful around the holidays or if you enjoy baking large batches of pies or cookies. You can also turn off the oven before the end of the specified cooking time; the residual heat will continue to cook the food for several minutes. It'll boil faster that way, and keep steam heat from escaping into your kitchen. If you boil water every day, an electric kettle can save time and energy because they heat up very quickly. For speed and convenience, many people try cooking meals where all the ingredients can be cooked together in one pot. Soups, pasta dishes, and rice-based dishes are great examples of recipes that can be made using only one pot. This may require cooking in stages, i.e., browning the meat, boiling the pasta, and then finally combining all ingredients in the pot to finish the cooking together. Once your pot has reached the boiling point, turning the heat down will save energy by allowing the heat trapped inside the pot to do most of the cooking.  Leaving your soup to boil increases the possibility of scorching food at the bottom of the pot, so lowering the heat can have more than one benefit. Cooking outdoors is a great summer pastime, but you can grill year-round if the weather permits. Cooking over an open fire or charcoal grill won't add a dime to your energy bills, and you can cook large amounts of food for use throughout the week.

Summary:
Open the refrigerator or oven door only when absolutely necessary. Thaw frozen foods in the refrigerator. Match the size of the pot to the stove top element. Optimize your preheated oven by cooking several items in a row. Use a lid when boiling a pot of water. Try one-pot cooking. Put soups and stews on simmer. Use a charcoal grill outdoors.