Write more and reflect on what you've written. I've come to have a dev diary where I reflect on what I did the past day/week, and note things I ran across that did/didn't work out so well, and why.

That said, if all you're doing is greenfield development I don't think it's actually possible to improve without third party input. A major goal of good code is maintainability, and that can only actually be known in hindsight.