Okay so I have this problem. I've only ever been able to finish two stories. One was complete crap and the other was a two page short story in which a girl meets with the grim reaper as she dies in the cold. That's not to say I don't have ideas. I have so many ideas it isn't funny. they can falling off me like sand at the beach. But I can never focus in on one idea. For instance, right now I'm working on an idea where a man writes a book but it's stolen by another man who publishes it takes the credit the fame the money, uses that to steal away his wife and their child and everything. He meets a kind of witch/demonic character whom sends him into the storybook world of his book as the guy changed it and wrote it. See, I love this story idea from just jotting that out, but I'm wavering now in the midst of chapter 2 and I'm doubting I'll continue it.
When this happened to me, it's because I didn't know where to go next. I'd write up to chapter six then quit because I'd get stuck on what the next scene should be then I'd just give up. So I started outlining more. I have every chapter laid out by scene (though not super detailed -- just a generic "they talk about this, she goes there" kind of thing). Using this method, I've completely planned out two novels, in the process of planning a third, and I'm halfway through writing one of them. Once I finish, I'll move on to the next. This doesn't work for everyone obviously. But it worked for me!
I'd agree with Lea'Brooks. Plan the whole thing to as much or little detail as you like. If you've got your character development etc. all sorted out, you can definitely jump into a chapter/scene in the middle of the story which you know will happen, and if you're still uncertain about other areas, brainstorm. Just don't allow yourself to get into a brick wall of not knowing/being able to write ANY of what comes next.
Don't be afraid to write scenes out of order, either. If you next want to write the scene where he meets the witch, just do it. It doesn't matter that the scenes leading up to that moment haven't been written yet. The more scenes you can get written, the more your story will take on a life of its own. Think of it a bit as a jigsaw puzzle. Fit it together in small increments, from all over the picture. Once you put that last piece in place, your picture is complete. Readers will read the story from beginning to end, in that order, but you don't have to write it that way. Follow your inspiration, and write the scenes that excite you. The transitions can come later. The most important thing is to stick with it, and don't quit till it's done. Keep thinking about your story in every spare moment you have, and let the vision grow in your head. Let the story problems begin to find organic solutions. If you get in the habit of abandoning stories whenever you hit a snag or get 'bored,' you'll never finish a piece to publication standard.
I agree with everyone above. Have an outline of where you want the story to go. If you're averse to outlining, it can be as simple as plotting points on a line so you know the events that will happen, leaving the rest up to the whim of the moment. To wit, you might have your idea, but now it's time to turn it into a story. Give it some thought before writing - and of course, outlines are as malleable as you want them to be.
Ditto to all of the above. Some people CAN successfully write on the spur of the moment, making it up as they go along. Other people KNOW for a fact they CAN'T do this. But then there's a third type of person, like a couple in this thread - me included - who THINK they write on the spur of the moment, but can't. I'm only writing my novel now, with any confidence, because I drew up an outline. Try the seven-point plot system. Get a pen and a pad of paper and fill in the blanks from these points, taken from here: http://csidemedia.com/gryphonclerks/2012/12/16/dan-wells-seven-point-story-structure/ 1. Hook In the hook, the character is in the opposite state from the state they will be in eventually. For example, if they are going to end up strong, start them out weak. (Harry Potter lives in a cupboard under the stairs.) 2. Plot Turn 1 Something changes that puts things into motion. New ideas, new people, a Call to Adventure or inciting incident, starts the movement from the situation of the Hook to the situation of the Resolution. (Harry Potter learns he's a wizard, enters the wizarding world.) 3. Pinch 1 Something goes wrong that forces the character to step up and solve a problem. (Harry Potter and friends fight the troll.) 4. Midpoint This is the point at which the character moves from reaction to action, decides to move towards the end state (knowingly or otherwise). It doesn't need to be in the middle of the story. In a mystery story, for example, where the midpoint is deciding to take the case, it can come very early on. (Harry Potter decides that people who suck blood from unicorns must be opposed.) 5. Pinch 2 Something goes very wrong, much more so than in Pinch 1. These are the jaws of defeat from which victory must be grasped. Mentors die or vanish, allies prove unreliable, plans fail. (Ron and Hermione fall to the magical traps on the way to the Stone and leave Harry Potter to go on alone.) 6. Plot Turn 2 The character receives the last piece needed to create the resolution. "The power is in you!" is a classic Plot Turn 2. Grasping victory from the jaws of defeat. (Harry Potter looks in the Mirror of Erised, and because his motives are pure the stone goes into his pocket and he knows that if Voldemort touches him it will harm Voldemort, not him.) 7. Resolution This the climax, what you're leading up to, what the story's about. This can be plot or character. For example, the character makes a moral decision and becomes a different person from the person they were when they started. The problem of the plot is resolved. This can be a state rather than an action (example of Poe's "The Telltale Heart", where the resolution is that the narrator is insane). (Harry Potter defeats Voldemort.)
^^ I've always wanted something like that to fill in the blanks. I find that we must first learn and follow the guidelines before being able to break and deviate from them. Thanks for sharing, OurJud!