This is what finally got me to use itunes as my main music player, and to start rating songs. I have 5 stars set to play every day, 4 stars every week, etc. until 1 star will only play once every 3 months. Unrated stuff keeps going until I rate it. This lets me do the "shaping" I wanted to do a while ago, by, say, moving the vast amounts of Tom Waits down a few stars while leaving my favourite bands higher up.