I have a playlist on my iTunes, and therefore on my iPhone for listening to when I'm not in the mood for anything in-particular.
It's a Smart Playlist that gets all the songs that have a play count of less than 2, or no rating next to it. It makes sure nothing in there is sitting there idly; and means I can weed out things that don't belong (occasionally come across some freebie and it makes me think "WTF?!", so...delete).
Currently it's quite heavy with AC/DC, Aloe Blacc, Alanis Morisette and Thin Lizzy.