I look for a question that is "unusual" in some respect, one that goes beyond a mere FAQ and requires more than a link to a prior thread or a perldoc page. That doesn't mean that I might not think about FPing a post that is incredibly stupid but has received some outstanding replies that are ingenious in how they point that out. :)
That said, if a question that would otherwise be thought of as run-of-the mill has generated some especially thoughtful answers I will FP it. But overall, I rarely FP something myself, relying instead on the judgement of other, more experienced monks than myself to do so.
It helps to remember that the primary goal is to drain the swamp even when you are hip-deep in alligators.