I already voted up Matt's answer "Good Abstraction" but I wanted to add something.
Documenting it's all about explaining things. I'm all in favor of Doxygen and other automatic documentation tools but crude lists of functions in an API are just better than nothing.
If you want to have your code to be maintanable, describe your solution to the proper level of abstraction and refine that level up to the code so that it's obvious what it does.
