Are you using the rating feature of iTunes and your iPod a lot ? I can only speak for myself but I never used that feature at all because it seems to be a lot of work to rate thousands of songs.
There is however a way to rate songs automatically using a free software called AutoRate (Macintosh) that rates track automatically according to how often they have been played and skipped. The ratings get synced with iTunes which means that you do not have to rate all of your songs by hand to achieve this goal.
