Before there were design patterns, before there was the Internet, there was the code review. When I started at Microsoft in the late 80s, I could count on enjoying a code review every couple of months. Everyone on the team would print out what they'd been working on and go into the conference room. There's no quicker way to improve than having four or five smart, experienced coders read your work and give you "feedback".
For this presentation, I brought the source code for an iOS app I wrote a couple of years ago, checkov. We went through it line-by-line, and I explained my decisions.