I've been thinking about this for a while. (Correct me if I'm wrong) It seems like there's a pattern to follow in order to make a good RP game/story.
1. Study how RPs work.
(Read them, join them, interact in them)
2. Think of a main theme.
(What moves the story along)
\/\/\(more like one long step)\/\/\
3. Know how it would work.
(Think of what everything/everyone should be able to do, and why)
4. Psych up for it.
(Get ready)
5. Start it and use your main theme.
(Do what you imagined should happen in your story)
6. When the main theme no longer applies... Wing it!
(It can end there, or it can become crazy... or crazier...)
Durandal
I would say that step four deserves to be in there quite a few more times, as it is important to keep yourself mentally focused throughout the entire process.
Other than that, I sort of disagree with steps five and six. You don't just kinda hope everything works out the way you want to. As GM, it is your responsibility to make sure the world makes sense, to ensure that the plot continues and develops in a way that the characters understand and is believable and relevant. You don't simply start it off in a general direction and hope it works. That's just asking for trouble.
Demecas
it is like this. you are god. you have set a story for these chars to follow. how they get from one place to annouther is up to them but they will always end up where you want them to no matter what they do. if they try to cross the sea instead of going to the port town on the land you can have a giant storm the wrecks them just outside the town. you have to guide them. not just hope it works.
ginpu
I'm agreeable with some of what Durandal says and some of what Demecas says and even some of what Zymeth says.
Studying other SUCCESSFUL rps does help you understand how one works.
You do pick a theme though that theme is more like a time period, what kinds of classes are allowed, items to be used, tech or magic, and importantly, the playable races.
As a GM you do need to be prepared. Player's will throw you off at times with their actions. You just pull out your huge weapon and smack them with it when they get overly stupid and ridiculous. And although I am against public humiliation, sometimes its the only way. Especially if someone follows that player's footsteps.
You need rules. Open ended RPs die fast. You need rules, level caps, skill caps. You have to restrain the characters. The object is that they get better as they move along, leveling its called. You can't start out all uber, if you do, just cancel the RP, it's just gonna die anyways.
You need a story this goes with the main theme. Like Demecas says, it has to be believable. The story is the biggest thing. It's basically the skeleton. Know what your goal is, and have an outline of the whole adventure. Details, IMO, get filled in along the way. Players make things too unpredictable, so the details are added along the way. This is probably where wing it would come in.
Herd the players if you must. If you've tried subtle things to move the players along, such as Demecas gets this feeling in her gut that the group is going the wrong way. Demecas the player would tell the group this. Should the group choose not to listen to her intuition, then you employ the epic level fire giants who chase them the right way. And you need to make it clear that they have no chance in hell at defeating said giants. Otherwise they are likely to try anyways, regardless of their level.
So all in all, you are god. Move the story along, move the players along, fill in the details, stop the players who want to be stupid, a good stomp tends to work, and continue on with the story. Details in the story happen as the story moves along.
Oh yeah, and listen to the players if they are griping or having a hard time with your RP. Don't just throw your hands up and give up. As god, you don't even have to have a character in the game. Though is does help since it let's you give information to the players and move them along.
Oh another helpful thought, everyone should be on the same side. It shouldn't be a good vs evil thing. You can always have good and evil characters together in a group. They just need to have a common goal.
Zymeth
So much info ... But I don't have time to read it. See ya!
... But I don't have time to read it. See ya!