This also works when grilling stuff or doing it in the oven. You can switch off the oven a good 10-15 minutes before the end for non-critical stuff i.e. not cakes or souffles or whatnot.
Also, another tip is to get a microwave with an oven/grill in it. The oven in ours heats up in no time and can have some stuff cooked before the normal oven is even up to temperature.