One thing that helped me a to was to build a system myself and maintain it for a few years. That way you see what areas come back and cause you trouble over and over again.

If I didn't understand a bit of code I wrote a few months ago, it was probably too complex and worth refactoring. As it was my own system, I couldn't blame anyone else's poor design decisions.

I noticed that keeping the logic as close to the database was generally the way to go. Avoiding changing the database schema and making up for it at the application level was basically adding technical debt.