Documentation is key. A couple of our volunteers have offered to help out and Roberto's already done some work on internal documentation (his PL/SQL vs. PL/pgSQL document for instance).
The problem won't be our understanding of the importance, but rather execution. This is *always* the problem with documentation in my experience, because good documentation requires input from developers (even external documentation) and most developers have too much on their plate and have to prioritize their work.
With a volunteer-based effort like OpenACS we need to work hard to discipline ourselves to make sure we do get all our changes documented.
We can start with the code itself, I pepper the code with little "DRB" comments (my initials) documenting my changes, usually with at least a little bit of background explanation. For minor changes this is sufficient, more major and systematic changes need to be separately documented as Kapil suggests.
