Learn how to evaluate trade-offs. Not everything is black and white. Given two alternatives, both of which have advantages and flaws, learn how to decide which one is better (and why).
Learn a sense of... I want to say "taste", but it's more concrete than that. Learn what the bad ideas are, and why they're bad. Learn a sense of "that doesn't feel right" that's technical, not just aesthetic.
Learn what it's like to maintain a two-decade-old code base of a million lines or more. See what the problems are. Think about what you're doing when writing new code that's going to create some of those problems.
Learn to document what you're doing to an appropriate level of detail. Take the time to do so.
...and appropriate level of detail doesn't mean every detail should be documented. Sometimes it's better not to document some technical details (which will change anyway, and docs will be obsolete soon).
Learn a sense of... I want to say "taste", but it's more concrete than that. Learn what the bad ideas are, and why they're bad. Learn a sense of "that doesn't feel right" that's technical, not just aesthetic.
Learn what it's like to maintain a two-decade-old code base of a million lines or more. See what the problems are. Think about what you're doing when writing new code that's going to create some of those problems.
Learn to document what you're doing to an appropriate level of detail. Take the time to do so.