I like to write down parts of a thought as I'm experiencing it, like others in this thread have alluded to. There has to be some sort of predicate for that to happen though, so a lot of the time I go back to old writings or collections of words, whatever, and change phrasing and whatnot to be cohesive with the track. I do enjoy writing about a pre-selected topic however: that is to say giving myself specific direction helps me to think of words sometimes.