Contribute to open source projects. And make sure you follow the style guide for that projects. (The style guides is often makes professional's code look professional.)
Come up with your own project, and it doesn't have to be a complete new idea, but if you try to create a real project, you'll run into problems or issues force you to learn new things.
If you haven't done so already, get an actual job where there are senior developers that you can learn from.