I know that at one point ovens probably took a long time to heat. Such is not the case anymore (and may not have been for the past 30 years if you take a look at my oven). When you are ready to bake or cook your food, simply turn on the oven and put the food in. Set it for the time needed and then when that time comes check to see if it's ready. Chances are it will be! Half of the time I forget to preheat it anyway and turn it on when I put the food in.
If you have a light in your oven, check your food without opening the oven. This will keep all the heat in and checking the food won't mean a longer cooking time.
So what are you waiting for?! Go make something and try it out. You'll save energy, time and money!