I'm of the school of thought that a roleplaying game session is a collaboration of the players and the Game Master. Together, they create the overall story. The players control the protagonists and the GM controls the antagonists and other characters. The players aren't intruding on the GM's storyline. There is no one player's character that the story revolves around, it's every character's story.
Extending this line of thought - that the GM and the players create a story - one can see that the campaign is the entire story. And like a story it has a beginning, a middle, and an end. Which gets us to the central point of this week's screed, plotting a campaign.
Sure, your campaign can go on and on, and there's nothing wrong with that. You're not limiting yourself to a solid storyline and can add on any tangent you wish. But with a limited campaign, one that is plotted out, you always know your goal, the final destination. This allows you to add foreshadowing, and lots of it, into your campaign. You'll be able to show little pieces of the overall puzzle into the adventures, that once the players get to the end, they can look back and realize how the total picture looks. This style of campaign is called an arc, and the overall story is called the story arc. There are two campaigns I know of that employ this technique, mine and Rick Jones'.
I originally got the idea for this style from Rick, who was inspired by Babylon 5. The way Babylon 5 was developed was before the first episode was even scripted, the series creator knew where the series was going. He knew the story would take five seasons and knew what basically would happen in each season, perhaps a page or a few paragraphs for each season. Then for each season, he wrote one or two sentence descriptions of each episode. And before each season starts (and as the first part progresses), the scripts for each episode is written. At the end of the first season we can look back and say, "Oh yeah, that was what Delenn's art project was." This is what you can do for your campaign.
You start with a story you'd like to tell. Let's go to the ol' comic book shelf and dig out Zot! #1 through #10. Basic story: There's a doorway at the end of the universe and a church as a key to it. Someone steals the key and Zot gets a hold of it (for a while). A group of people are trying to overthrow the government of Sirius IV by opening the door and returning, being hailed as the new Messiah. Eventually Zot and the gang realize this and wind up trying to stop the coup. Let's make this into a Shadowrun campaign.
Here's the basic storyline: An AresSpace exploration to Mars discovered something odd. There's a doorway there standing in the middle of nothing. It's made of wood, has a frame, hinges, doorknob, and keyhole. But no key. The key is on Earth, in Atlanta - the Atlantean Foundation's main building. Our players are the high ups in Ares, the Immortal Elves, and a cast of dozens. One of the Immortal Elves is out of favor with the other Elves, and he knows that the doorway is a portal to Earthdawn's Parlainth. If he can get the key and pass through the doorway, he'll be able to regain power. We'll get to the specifics later in the campaign. Along the way, Ares gets word that something's going on with that doorway their robot drones discovered. And the Immortal Elves get wind of what this rouge IE is doing, and they try to put a stop to it. This key comes into and passes out of the lives of the shadowrunners who eventually realize what's going on.
Now that you've got the basic storyline down, you work with your players to develop their character's backgrounds. Let's say that one of them has a character that used to be in the military but is now out. You collaborate and determine that the reason why he left the CAS military was that he was an officer and had an affair with a junior officer who was married to the PC's commanding officer. What you, the GM, know is that later the junior officer and the CO eventually divorced due to this event and she is now in charge of security at AresSpace Needle, the orbital station that the runners will have to go through to get to Mars. As the GM, you help the character's background fit into the storyline while keeping the character the same type of character the player wants to play.
At this time, you should also be writing out the major plot points in the campaign. The characters have a friend, another runner ('Vic'). Around the second or third adventure, Vic disappears. The runners hear about a run Vic pulled against the Atlantean Foundation. A run or two later, they get a message from Vic, but when they go to the meet, they get the key. Vic gets snagged by the Elfs and the Rogue Elf starts looking for the key and hires another runner, Nine, to track down the key. (We'll divert from Zot! here.)
Somehow, the runners loose the key during missions that have nothing to do with the overall picture. Nine doesn't know that they've lost the key, and it's Vic who's stolen the key from the runners. The runners go on a run against the AF, and while they're there, they discover a file that has the information on the key and Vic and how Vic escaped, but only during his being mindwiped so the IE doesn't know if they've broken Vic or not. Which is a shame, because at this point, the runners don't know if they can trust Vic either.
Next thing they've got to do is head up to the AresSpace Needle and try to save Vic before he heads off to Mars, but the Rogue Elf is going to finally catch up to Vic there and steal the key and head to Mars when Ares finally catches a clue and sends them off to Mars after the Rogue Elf to get whatever is behind the door for Ares, and the grand conclusion happens on the desolate Martian landscape. Someone might go through the door, and you can have an epilogue game session where the runners come back with enough power to tell the Atlantean Foundation, Ares, and the Immortal Elves where they can stick their damn key.
Now as you go, you can plan out individual runs, add NPCs, and throw in a few subplots. You just have a path that the campaign will most likely go -- although if the final session takes place on Mars, the Needle, or Space Center Houston, hey, it's just a game.
Now that wasn't too hard, was it?