Your cooking and baking depend a lot on your oven's ability to regulate heat and accurately hit the target temperature. You can prepare your oven so it's properly calibrated and has more even heat distribution using just sugar and a pizza stone.
Photo by Thomas van de Weerd.
These tips come from Jeff Potter's excellent book Cooking for Geeks, which is all about the science of food and ways to experiment with cooking.
The first tip involves just placing a thick, heavy pizza stone at the very bottom rack of your oven to even out the heat and improve the oven's recovery time. When you're baking, for example, and open the door to put cookies in, the pizza stone acts as a thermal mass to recover faster for the hot air lost from opening the door.
Check your oven thermometer's accuracy using sugar. Potter writes:
Pour a spoonful of sugar into an oven-safe glass bowl or onto some foil on a cookie sheet and place in your oven, set to 350°F / 177°C. Even after an hour, it should still be powdered. It might turn slightly brown due to decomposition, but it shouldn't melt. If it does, your oven is too hot. Next, turn your oven up to 375°F / 190°C. The sugar should completely melt within 15 minutes or so. If it doesn't, your oven is calibrated too cold. Check to see if your oven has either an adjustment knob or a calibration offset setting; otherwise, just keep in mind the offset when setting the temperature. Note that your oven will cycle a bit above and below the target temperature: the oven will overshoot its target temperature, then turn off, cool down, turn back on, etc. It's possible that your oven could be "correctly" calibrated but still melt the sugar when set to 350°F / 177°C due to this overshooting, but it would have to overshoot by about 15°F / 8°C.
That done, you should be able to cook and bake more precisely and accurately.