As an example, I used to mark emails as important if I wanted to remember what was in them or respond to them. No more. Now I just create a small Wiki page and put a note in my Planner. No more month’s old messages sitting there.
Of course, marking the email was probably the wrong way to do it in the first place, but, like Kai Grossjohann, I was using my INBOX as my TODO list. Using an emacs-based Wiki, I actually have a somewhat disorganised place to put things.