With conventional ovens, keep the preheating time to a minimum. Unless you're baking breads or pastries, you may not need to preheat the oven at all.
Food cooks more quickly and efficiently in ovens when air can circulate freely.
Don't lay foils on racks. If possible, stagger pans on upper and lower racks to improve air flow. Use glass or ceramic pans in ovens. Do not open the oven door often to preview the food. Watch the clock or use a timer instead.