To me, a project log is to documentation as Twitter is to blogging--it requires little effort but over time provides useful pointers to where you were at the time.
Originally I used a "one way personal wiki" to keep track, like this: http://code.rancidbacon.com/ProjectLogArduinoUSB
But when I lost the server it was on I decided to re-implement it on Google App Engine and make it available to other people, thus was born Labradoc, here's an example project log: http://www.labradoc.com/i/follower/p/project-sms-text-scroll...
It's kinda a "commit log for your day".
Yeah, it's really helped with those "now, what was that command line I used?" and "how did I solve that problem last time?" type questions too.
And not just for yourself. It also helps other people pick up "incomplete" projects you might have lying around that you cough haven't got around to documenting yet.
I started out with just writing single line entries but because they proved so useful it encouraged me to spend a little more time writing longer entries. (Although I often end up with a whole bunch of browser tabs open as a result. :) )