I for one will often (and usually naturally) befriend the most talented developers I work with. Talking code often and bouncing ideas off of them will really help you see where you're lacking or where you can improve.

Learning new languages and different paradigms can also play a huge role in how you approach future problems. Go to glot.io and play around with a handful of languages you've never touched.

Read up on various best practices and style guides. Read blogs on architecture, design patterns, etc. I read probably 5-10 hacker news articles daily, but I also watch many videos on youtube such as pretty much anything by Martin Fowler, Rob Pike or any of the other greats.

Code stuff outside of your comfort zone. If you're a web developer, try writing some games. If you're a game developer, write a web server or a blazing fast load balancer that handles TCP/UDP sockets and HTTP. I code random shit all of the time just to gain some exposure even if I'm never likely to do related work in the future. I probably have a few dozen Go programs I've developed just because I was curious how something worked.

I've been programming for over 17 years starting with C++ when I was 13. Over the years, I've developed stuff in C/C++, Python, Ruby, Lua, C#, Java, JS, TypeScript, SQL, Dart and Go for the past 3 years. Each language has given me new insights in how to tackle problems that knowing say just C++ alone would not have lead me to.

TLDR: Exposure!!, Books, Blogs and LOTS of bouncing ideas off of other developers.