Leave the oven door closed. The more you open it to check on cooking food, the more heat you let escape, and the harder your oven has to work to stay heated. If you’re cooking a dish that has a long cook time, such as a turkey, put the dish in as the oven is heating up. Then turn the oven off and allow the heat in oven to finish the job. If you’re making two dishes that both require oven cooking at about the same temperature, put them in together.