Don't Preheat

For some things, like breads and pizza, you have to preheat the oven so it cooks the outside with a crispy crust, without baking the softer insides to. But for other stuff, you can put whatever you are cooking right away, and let the cooking begin. No need to preheat the oven.

When your dish is almost finished, turn the oven off and let the dish finish cooking using the heat of the oven. If you leave the door closed, the heat will remain inside for a surprisingly long time.