The thing that taught me the most was writing an application and having to maintain it for 4 years. You see where the pain points are (as you end up revisiting them often), and you have no one to blame but yourself. You learn from your own mistakes.

I also learn from other mistakes as well. I has a habit of inlining conditionals in python -

   if False : continue 

until I noticed that the same style made reading someone elses code harder. I didn't notice it in my own code as I was more familiar with it. When I did notice I stopped doing that.

Keeping the logic as close to the data as possible helps as well (ideally in the data schema if possible). I think Linus's quote about bad programmers worry about the code, good programmers worry about the data and their relationships is true for most of not all levels of the stack.