Bought a $7 oven thermometer on a whim. My oven's '220°C' is actually 195°C, and it swings ±15° through the bake cycle. Years of 'why is this recipe taking longer than it says' — solved. Cheapest upgrade in my kitchen. Gary (the starter) and I both recommend it.

For me it was learning to actually salt in layers instead of all at the end. Tasting as I go changed everything. What's the single thing that made your food better?