For me, the key to prepping for a campaign is keeping a solid core of information that you use to guide you as events develop forward.
Broadly speaking, all I think you need to prepare three main things:
-The “Big Bad” –the oppositional force, the enemy, the villain. What do they want and why do they want it? How do these desires lead them into conflict with the heroes? And what happens if the heroes do nothing? Make the your bad guy strong, give them a clear motivation and well defined perspective.
-The “Seed” -the core facts upon which you build your story, or you might say: “what’s really going on." Ideally, the seed is not so big as to bog you down with details, but full of crucial information that you can use to build outward where you need to build out (if A is true and B is true, therefore C, D, and E must also be true) Keep the underlying information succinct and dynamic.
-A Sense of the "Hooks" (and also "Pins") – In the "Hero's Journey" sense, this might be called "The Call to Adventure". The Hook involves how the story "appears" from the outside and how it pulls the characters in. A Hook is like a stray thread (or threads) that, if you pull hard enough, could unravel the entire proverbial sweater (and potentially tangle you up in yarn!!!) I would offer a "Pin" as a twin to "Hooks" - Pins are the way a story deals with characters that ignore or fail to deal with problems.
These three areas comprise a dynamic core that will keep you afloat as you navigate the story forward.
Me and my boys play weekly. Personally, I enjoy the rhythm of a weekly encounter. Every week is a little different; sometimes you never get to anything you had prepped or sometimes the follow through for the next session is obvious. But, broadly speaking, my week tends to look like this:
Monday is Nerd Night. Tuesday I wake up with the afterglow. But that is short lived. Very quickly it turns to "How the hell does this move forward?" Wednesday I think about how stuck everything seems. Thursday I watch tv. Friday I watch more tv. Saturday I clean the house and start trying to figure out what I find intriguing about our current fictional circumstances, what loose ends might be left over, what broad questions/curiosities I have about the players' futures... Sunday I start thinking about the nuts and bolts of what I consider to be the likely direction for the next session. (Knowing full well that players might prove me dead wrong). Monday I have one last startling realization that suddenly shifts everything I have prepped.
Then it's go time!
I come at role-playing campaigns from the perspective of an adult with a busy life. And, naturally, this affects what I want out of a campaign. Perhaps it's better to say, it reinforces my desires. For one, I just don't have the time to invest in a meandering "sandbox" style campaign where the group wanders far and wide and does a little bit of everything under the sun. This approach is best when time is plentiful and/or DM's are scarce. On the other hand, a pre-packaged module (or an overly prescriptive original campaign) starts to feel like a drag on your day, an activity that takes more than it gives back. A role-playing experience, for me, needs to be compelling enough to clearly justify the activity (or, another way to say it might be: I need the experience of RPing to be it's own end, rather than having any traces of doing it with a "killing time" mindset - RPing because there's nothing better to do.)
So, assuming you agree with me that Free Will is the central cornerstone
of what makes RPing great... the question remains: how exactly do you
prepare for a game where people can, in theory, do anything?!? I say, reject either extreme and shoot for a middle way - plan, but try to plan simple and plan smart - and plan in a way that you can move the story forward no matter what crazy thing the players might try (or, perhaps better to say: move forward by riding on the backs of the crazy things they try!)
I mean, you have to figure out where the line is between comfort and discomfort, success and failure, and walk it.
Players, by their nature, will usually seek the comfort and security of their own percieved exceptionalism. They tend to build their characters with one thought in mind: how will I be awesome? There is nothing wrong with this. But as a DM, if you let your players lounge in this realm of comfort, your session will simply not be compelling. (Likewise, you cannot expect that players will enjoy themselves if all you do is beat them down and negate anything they are supposed to be good at.)
You've got find ways to get them to the point where they teeter on the edge, on the brink of disaster, where their abilities are challenged, but not negated... where their future is uncertain.
Becuase it is only when we walk the edge that the choices that we make truly matter - both for the players themselves and the world around them.
As a DM, you should be asking yourself: What do your player's stand for? What are they willing to sacrifice for success? What are they not?
You'll only find the answers if you can push them to the edge.