Little By Little
I am, in fact, running a Dresden Files campaign right now. We're nearing the end of our third "book" - we've arranged the game in books and I've got some things I've learned that might be of use to you. But as the title says, the way to do what you want to do is little by little.
First of all, forget about plotting RPGs. The plot of a story is the list of all the things that happen. But you can't plot, because you can't predict what any of the main characters will do at any time. Which is great. Because you have a bunch of smart people (your players) taking care of that for you, so you can concentrate on other stuff! All you can do is provide instigating events for them to react to. In other words, you can't plot, but you can plan.
By the Book - how the system can help you:
The Dresden Files RPG has some great features built in that are going to make some of this easier. It's got a great city-creation system that will help you make sure you've got a city full of interesting factions and characters. And I don't know if you've played FATE-powered games before, but you're going to end up knowing a lot more about the PCs than how hard they can hit something.
Begin by Ending - how to get started:
If you wanted to drive across the county, you'd never try to predict exactly what you would do at every step. But you would want to know what city you intended to drive to before you started, right?
Before the game starts, I talk with my group about what we're going to be doing in general - I make sure that I'm not set on an all-Nevernever-smackdown game if the players are more interested in playing cat-and-mouse with mortal authorities while hunting Lawbreakers for the White Council. Once we're on the same general page, I would do the city and character creation. Then, I have a fantastic resource at hand: The PCs and the city. Those are like a mad scientist's control panel, full of buttons to push, levers to pull, and dials to twist. The players have just given you the keys to the characters. It's up to you to drive them! Pick a big bad that will push those buttons - activating he Aspects of characters and city alike.
Now, jump to the end. Decide what would happen if the PCs weren't there. What is the thing that your players are averting? Make sure it will make the PCs take notice. It doesn't have to be the end of the world, but it has to impact the PCs somehow. Take this consequence of inaction as an example:
"If the PCs don't intervene, the municipal pools will have their hours reduced."
Some groups may not think that's worth getting out of bed to prevent. But if your PCs all grew up poor in this city and have aspects to reflect it, this may provide plenty of motivation.
Usually, of course, the consequence of inaction is bigger.
"If the PCs don't intervene, the swing voter on the city council will be a Red Court thrall,"
for instance. But bigger isn't necessarily better. Harry Dresden goes to tremendous lengths to prevent small tragedies sometimes.
Once you have a Big Bad and a goal for him / her / it / them, you can try to see how the Big Bad's agenda coincides with or crosses the agendas of the PCs and the city factions you defined above. That will give you some insight into what could happen in the future - but you don't need to get it all pinned down yet. You know the endgame and the primary actor. That's enough for now.
It's also possible to have endgames that don't have actors - inexorable threats like asteroids are essentially timers for this endgame:
"If the PCs don't intervene, the city will be blasted into a crater by an asteroid."
But villains are generally more interesting for you to play than forces of nature are. Asteroids don't cackle and tent their fingers.
One Bite at a Time - how to eat an elephant:
Here's how I handle planning a given episode: Take a notebook and make some columns on it with these headings: Who, Wants, So. Use a notebook. It helps keep you organized. Write longhand. It helps, I promise.
Now, take that notebook and write a name in the Who column. It can be your villain, a PC, a city faction, an NPC, or whatever. You might write "The Red Court" in your Who column.
Next, write down what they want in the Wants column. Wants should usually be short term - so the Red Court is going to put their puppet on the city council. Their want should be what they want next - like "The Red Court wants to get the councilman addicted to Red Court saliva."
Now that you know what they want, write down what they'll do as a consequence in the So column. They want to get the councilman addicted, so...."They arrange to have a fundraiser at a Red Court stronghold where they can get a little face time with the councilman."
Now you've got a strongly motivated action for the bad guys to take - repeat this until the "prods" for your upcoming game start to take shape, and then you're done with the "What's going to happen?" part of prep!
All you need then is to sketch out people and places you think will be important. Don't go nuts - you'll be wrong about some of them and don't want to waste the work. And a lot of games (and DFRPG in particular) make creating these things on fast and simple enough to accomplish on the fly. I just ran an awesome session with a rescue mission / fight in a burning, collapsing warehouse full of nasties that I built right there at the table while everyone else was grabbing a bathroom / soda break.
And that's it. By keeping this in a notebook, you can flip back a page or two and see if there are any loose ends from last time, or any previous time. If a couple of sessions back you said that
The mayor wants to make his mistress disappear so he encourages her to return to her drug habit, hoping she'll OD by herself.
but you never got around to dealing with that, you've got yourself a nice loose end to pick up and tug on.
Here are the advantages I have found to using this method in my fairly long-running DFRPG game (and others before this):
- You never need to envision the plot in its convoluted, interwoven entirety. You just need to plan the ending - what happens if the PCs don't get involved - and then take small steps from where you are towards where you want to go.
- The campaign builds itself organically. The past informs the future in a gripping, well-motivated story.
- You never have to railroad your players. The Big Bad will adjust his plans as his first attempts are foiled, instead of trying to make your players fit a cast-in-stone plot.
- You get to look like a genius at the end. I have had things I forgot about become hugely important when the players get a great idea about how to use them. Then they say, "What a great plant! It was so subtle!" I just smile and try to look like I deserve it.