Ask HN: What is the hand-washing of development?

I'm reading "Better" by Atul Gawande about how physicians and medical care can improve.

He cites hand-washing as one of the simplest and most effective ways to prevent complications from hospital stays. He writes about how it's universally effective at reducing infections, but still doesn't get implemented for dozens of different reasons (including laziness).

What is the "hand-washing of development"? What is a simple practice that, when universally and consistently braced by a development team, always results in higher quality output with fewer defects?

Put another way, what are we most often not doing that we COULD do and know that we SHOULD do?