Systematically removing code
It's easy to miss things when removing code, leaving behind unused methods, templates, CSS classes or translation keys. (Especially in a dynamic language like Ruby, without a compiler to help you spot dead code.)
I avoid this by removing code systematically, line by line, depth-first.
This is one of those things that seems obvious when you do it, but in my experience, many people do it haphazardly.
Source: Systematically removing code, an article by Henrik Nyh.