Ideally we preheat our ovens while we're mixing or preparing the food. But there are those times when we just forget and/or are short on time. So, here's a tip to cut oven preheat time in half... at least.
1. Turn on oven to broil.
2. Set timer for 3-5 minutes. (depends on your oven and how hot you want it to get)
3. After timer goes off, take off broil and set oven at desired temperature. Oven should give you the signal (beeping, or a light) within a few seconds to let you know that it is preheated.
Easy as that. My oven preheats in about 10-15 minutes depending on the temperature, so using the above preheat method, I can cut the preheat time to 3-4 minutes. Of course, I'd rather just remember to preheat while I'm cooking, but this trick can come in handy.