My friend Tyson works at a major insurance company. Recently he shared with me a technique that he uses there: He keeps a work journal to write down domain-specific things that he learns.
Journaling could include writing down a technique or rule of thumb used, a page of a book that was helpful in a certain area, or who to contact in a different department in case of questions in a specific area. Sometimes it could be something that went particularly well so that he can look back when times are tough and remember a time when he persevered. Other times it is something that didn't go as well as he hoped. He also does this to clarify his own knowledge so that when he needs information he has a place to look, and for potentially transferring that knowledge to other people.
I recently tried experimenting with this technique. I suppose that I like writing, so this came somewhat naturally. I liked separating this from other writing that I do because it seems useful to have it all in one place. I am just capturing knowledge that I have gained from working on a project. Writing this down regularly is really helpful in writing documentation later because you explicitly state what you have recently learned. This prevents me from being blinded to what I learned and taking it for granted. It might be obvious to me at the end of a project why the build works the way that it does, but along the way I needed to learn a lot of things and make various decisions. So this seems to be a good way of documenting decisions for later understanding and analysis of results.