Personally, I keep a timesheet on when I started and finished working on something, for instance

Tue 19/10/2010 | 15:40:00 | 17:10:06 | 1:30:06 | Looking at admin interface for sensor daetails. Trying to fix None issue, seems to be related to subclassed models. Adding code to twisted for CT settings

Turns out that one of my Django models is breaking the Django admin interface, and also that I have to have calibration details for the Current Transformer sensors we use (http://gridspy.co.nz)

Another one

18/10/2010 | 8:50:00 | 9:56:36 | 1:06:36 | Server maintaince, removed orbited, checked memory usage. Killed backup scripts, installed python 2.7

Naturally it is all in an Excel spreadsheet. :)