As I see it, the fundamental rule in writing maintainable code is that your code should be very easy to understand. This isn't as easy as it sounds, and you'll have to use all of the other techniques mentioned here to do it. It requires a certain amount of empathy because you'll have to learn how other developers see your code, and how it differs from the way you see it. A good way to get a grasp of that is to go back and look at some code you wrote a couple years ago.
Now, I suppose that it would be possible, theoretically, to write code that is very easy to understand and performs exactly the task it is intended for but which is also hard to modify in any way. I've never seen code like that, though.
