These small log files really help to keep things in perspective. However for longer plans I use Trello. It works as an issue tracker / planner for me so I can put the bits I find on the issue to the cards, or to my dropbox and link them.

When I finish a task, I put all relevant information to a folder in my "Old documents" folder, and put the card to the "Done" column. If the task is a project, everything is already become documented and archived on git at the end.