Often, when designing an adventure or, specially, a campaign, we have a preconceived idea of what we want to do, and start building a whole story from the ground up, and then we shoehorn the PC´s in, or request that the players create specific kinds of characters for the kind of game we are going to run. This, however, takes very little consideration of the players´ philias and phobias, and specially, it ignores what kind of game they want to play.
For this reason, it is often a good idea to ask players those very questions: what do they expect from this game, what is it that they´d like to see done, even if it´s on simple, abstract terms: for example, in a D&d game, do they want to play a game of high adventure and epic storylines, or a smaller, more down to earth game, with lots of side quests tied to specific locations?
Another thing that must be taken into consideration is the players you are dealing with. A friend of mine recently told me that after playing with some novice players (his girlfriend, a friend of his and his cousin, all of which were first-timers in gaming), he realized that the plot he had come up with was far too complex for them, and they were in over their heads. He reflected on the fact that, for a more experienced RP´er, the plot would have felt simple, and that they would have have no problem uncovering it if that was the case, but that due to their lack of experience, they could not follow what was going on behind the scenes. I pointed out that, for novice players, you needn´t complicated stories to surprise them or make them feel things are new and enthralling, and that often, something they can follow and get to the end of will feel satisfying enough.
Finally, you have to strike a balance between what you want to run and what your players want to play. If you are running something that has no appeal to you, you will soon lose interest in the game, and it will end up in a failure.
With all this in mind, the best way to plan what you want to do is, once you know what your players want, adapt what you want so that it fits both your desires and your players´. Making a rigid outline of the story won´t help. No matter how clever your plans are, your players will always break them in some way. For that reason, be sure to be flexible, and while you will need to plan ahead, don´t be afraid to improvise something out of the path. If you can be quick enough to make the players go back to the original plot without them feeling like they are being led by you, then all the better.
To sum up:
- Decide what you want to do.
- Learn what your players want you to do.
- Mix those two.
- Spice the result up with some good plots and ideas that fit your players.
- Be ready to improvise to correct the whole thing