Getting things done includes deciding what not to do. In my case, it's a matter of going through my todo list and striking off the well-intentioned tasks from two or three months ago that took a moment to write and that would take hours to complete.
I'm better off at accomplishing things when there are fewer undone things to choose from.
Janet Choi writes about the done list for 99u.
When you do anything you consider useful, however small a win it may be, write it down on your done list.
This can lead to the purposeful behavior of writing something on your todo list just to cross it off and mark it done. The system I'm using doesn't easily distinguish between the three states of "did and then wrote it down", "wrote it down and did it later", or "wrote it down and decided not to do it later". But that's a quibble.