Recently I read (don't remember where, probably on the XP list) of someone who used to keep a failed unit test at the end of the day. The next day, it's obvious where you stopped the previous day, and it's obvious where you need to start working.
Usually I leave everything checked in and the tests at 100%, but Monday afternoon I tried the "failing" approach. After a complete day away from the code (project management training) it actually worked according to plan this morning. And after the first problem was solved and the tests back at 100%, I was up to speed on the coding and the problem domain.
Now I can perhaps stop writing myself post-it notes for the next day