I like taking notes during debugging, it helps a lot. I use an org-mode file, per project, for it. It can contain links to the code, so I can record where I have been while tracking down a fault. Very handy when working on foreign codebases, it's slowly growing into something like a travel guide. It can also have snippets, checklists, etc., anything one could want. When I fix an issue, I write a summary of what was wrong and why I think my fix is the correct one. After that, I change the TODO state into DONE and feel good about myself.
Especially useful for projects done in spare time, where it helps tremendously in getting back in context, even after longer intermissions.