Things are always the way they are because they got that way.
How things got that way
Andy Hunt's (Pragmatic Programmer) blogs a quick post about how "things are always the way they are because they got that way".
I always try to look at what I am checking in and ask "does it improve or weaken the overall codebase?" I've never come across a situation where the honest answer is that my contribution is neutral; I'm either improving the code for the next person who checks it out or I'm making it worse. If the team all make the code just a little worse each check-in it soon adds up.