I came up with a sort of formula for producing Dresden Files sorts of situations a while back. It helps me, and it might help you as well.
* Set up -- What's going on to hook the PCs? Think of what the beginning of the story might be like, just before the players get to do stuff. Basically, you're looking for something to make them want into the story.
* Things get worse -- You know how Harry's life is never simple? Well, this is part of that. It's a complication that pops up after a little while to make the characters' lives more difficult.
* Life gets even worse than that -- Let's face it. Suffering characters are interesting characters. Here is where you introduce even more complications.
* Conclusion -- Don't decide how you think things will end. That's madness. Instead, set rough victory conditions. Let's say that stopping a ritual decides victory or failure. What sorts of actions might stop the ritual? What happens if they do stop it? If they don't?
Now, cross reference this with the list above. During the introduction, you might set things up to include a basic set of skill rolls. Add another scene where you bring in Aspects.
Then, life starts to get worse. You might introduce social conflict here. You might also introduce more complicated skill rolls.
Etc.
