I think this one goes without saying in almost any discipline. Having a mentor — someone who is wiser than you in some aspect of what you are doing, and hearing out their suggestions — will always stretch your brain. Of course, it can take a while to find someone you click with and who also has the time for you.
While you’re hunting for a mentor, you can get the next-best thing by reading the writings and the code of “famous” software engineers. Books like Programmers at Work and Beautiful Code are great for this sort of thing. And tons of very clever people write open-source software that you can peruse.
While seeking a mentor is a somewhat obvious way to improve, inverting that relationship by looking for a mentee — even when you’re very green yourself — is, in my opinion, a great way to improve as well. Einstein’s old adage of “You do not understand something unless you can explain it to a six-year-old” comes into play here.
I’ve had quite a few opportunities to teach intro-level computer science and software engineering courses, and these experiences clarified a whole bunch of things for me in a hurry. As I sought explanations and examples that would click well with an entire class, I started to learn what methods of organizing programs in the small would communicate well across a broad audience. I try to carry those lessons over into “real-world” projects, so that other developers can (hopefully) make the appropriate modifications to work I’ve done regardless of their skill level.
Plus, working with people who are just starting their journey is refreshing. Their optimism and the breadth of (often strange) approaches that they take to problems that to us are old hat is a lot of fun.
So, get out there! Give, take, interact. Humans are the interesting part of this craft, after all.