^Well that seems to work when you're just manufacturing short stories. Occasionally, anyway.
There seems to be two major approaches to coming up with story ideas (I'm generalizing of course), and that is basically either character-driven or plot-driven.
For character-driven stories, more likely that not, the author usually has either 1) a pairing in mind which they want to explore (or put together, or tear apart...), or 2) they just want to write about a specific person or people.
With that in mind, inspiration can come from just about anything. In our case since we're writing about the girls, media appearances and the like can give us ideas, or situations in our own lives might give us places in which to throw the girls into. Which I theorize is why there are so many high school fics around in general when it comes to ficdom (not just H!P, everywhere else).
Then there's plot-driven, in which case the storyline comes before you start picking roles that would fit into the plot. Let's take my own crazy story, AA. My premise was very simple. After having read through the entire fanfic section at the time, I noticed that there were no pirate stories! Therefore I wanted to write something with pirates in them. Next would have been "what would the pirates be doing???" and "what do pirates have to do with the story??". And it all went on from there. Ninjas, conspiracies, and Highlander jokes came later.
To put it simply, either you pick the characters first and form the story around them, or you pick a story first and fit characters into them. That's the main idea, but there are different ways of approaching the whole business. Just my two cents.