I pretty much know when I wont be creating anything good for the day. I'll sit down at the piano in the morning, mess around and try to think of something clever, and if I dont have anything, thats the way it is the entire day. On any given day though I might be listening to this kickass song, and I'll simply get up and sit down at the piano, the boom - I've got something good. I think it just requires patience when creating new material.