Read "Code Complete". Read "Mythical Man Month." Maybe read "The Pragmatic Programmer." If you're interested in learning more about testing, read "How Google Test Software." If you work on a lot of other people's code, read "Working Effectively with Legacy Code"

Good unit tests are absolutely crucial to any kind of future refactoring effort. They are helpful at finding regressions and give you some confidence that you do not break the paths that are tested. If you work in a team larger than one, code reviews are a must. Code reviews allow both participating parties the opportunity to learn, improve the overall codebase, and catch bugs. I also think writing code comments and documentation is absolutely important. Mostly so the "next guy" understands why the code is doing what it is doing. Most of the time the "next guy" ends up being you.