What were the first Programming Best Practices you learned?

There was an awesome tweet from Dan Abramov that asked: "What are some “best practices” in programming that you learned as a beginner or in the first few years? (Both the ones you still agree and disagree with now.)"

I loved reading through this thread. It's a real gem. So, I'd like to also get your opinions.

What were the first Programming Best Practices you learned, and are they still valid now?