Besides experience mentioned in the other replies, I'd recommend reading open source code. Search for open source projects related to the programming language o

Besides experience mentioned in the other replies, I'd recommend reading open source code. Search for open source projects related to the programming language of your choice. Other ways to find good projects to read are to look through the codebase of whatever framework you're using, if you're using one. Are there any dependencies/libraries you always reach for? Read through that code as well. Learn design patterns to properly decouple parts of your system. The Gang Of Four book is considered the best book on this topic.