Pre-heating.
Myth unless you are baking. I never pre-heat for meats or vegetables etc.
Mostly - if it ain't cooked by the required time - I just leave it to cook a little longer!
There are however a couple of items that need pre-heating to get the best results - usually puffy things like Yorkshire puddings, puff pastry, and sponges need to go into a hot oven, and roast pork gets much more crackly if you heat it fast for the first 20 minutes, and then turn the heat down.
More cooking tips next week on Ovenspotting