Write everything down
Write everything because you will forget. The older you get the quicker you forget everything. It is just how an entropy of the Universe erodes your biology..
So, write the principles down and link related ideas together with hyperlinks. Wiki (and structured text) is a milestone in evolution of knowledge representation.
Write down all important conversations - it is much easier to re-read a discussion than memorize it. Thait is why HN is such a huge success.
Write all design decisions, big and small, and link them together to form a big picture. Write whys first and hows second. Remember that a layered set of languages (or a Duck-Typed interfaces) is the way to manage complexity. So, write that layered description of complexity in English first.