Since the site just kicked off and I wanted at least one blog post on the subject of writing, I was going to cite a study about immediate gratification and how it ties into writing, or any art subject for that matter, so let me try this without absolutely any studies or science to back it up.
With any complex machine it takes a bit of understanding to keep it running, and running right. There's really two frontiers we as humans haven't mastered: Space and our own brains. Considering that there are more neurological connections in the human mind than there are stars in our galaxy, it's not so difficult to understand why we have so much trouble with it. Some people gaze at the stars in awe, and some look at MRI images in awe.
Almost any writer, myself included is going to have trouble keeping on task and staying interested in what they're working on. I think there's a few ways around this. We're not talking about writers block, but simple boredom. Think of it this way, you've got in your mind a complete map of your story, where it's going, the characters, the interactions between the setting, characters and the general narration. You know how this is going to go. You absolutely have it figured out. One day, say 50,000 words in and halfway through the story itself, you loose interest. Why? What happened? Suddenly you're thinking about another story, or your mind is preoccupied with something completely unrelated to filling up blank pages.
I'm not proposing an answer here, but I might have one or two solutions. First and foremost, don't tell anyone what you're doing. If you've written a few things before people around you are going to take it for granted that you'll be writing again. They'll ask about those things, and it's too easy to spill the entire story to them. But once you've exposed the whole tale to them, and they kind of look at you like you're onto something, maybe something they would pay the cover price for, you've weakened your resolve. On some level, the brain thinks 'okay, I got a reaction'. That's great and it might make you want to go home and put down another five or ten pages before bed, but that feeling slips away (speaking from my own personal experience). You tell somebody else the entire story... well, you've just told your story twice. And instead of having to write it, you just spoke to someone for five minutes and they gave you that 'good job' nod. It's gratifying, and it kills the will to write.
The difference is that you shove the story into their brain instead of letting them experience it for themselves, with their own imagination while reading the story, at their own pace. You might not think there's a big difference, but I'm confident it's the difference between a completed work and one that's abandoned halfway through. So the trick, the one I employ, is to give as little information as possible. I don't want to tell people 'I'm not talking about it right now', for reasons I'm sure you can infer. But I do give them the vague abbreviation. Take Souls of Silica, though I didn't recognize this trick when I was writing that story, my response today would be something like 'a shooter who wiped out a town, sometimes without seeing who he was shooting'. That's not nearly as engaging and romantic, and it could probably be put better in so many words, but that's the gist.
Or you could lie to them. I've done that before, particularly when I'm right at the crescendo of the story and don't want to risk spilling anything at all for the sake of my own work effort. I don't do it to be surprising, or because I dislike somebody, I do it because I'd rather you read it then let me give you any aspect of it (On a personal note, and I don't know if I'm the only one like this, but I'm better at speaking non-fiction, things that actually happened to me, than I am writing them. Vice-versa for fiction. If you're a fiction writer, I hope your the same way).
The other trick to not being bored is to manage the time and effort you put into the story. Some say you should only write so much a day, and I recommend that for anyone who hasn't been doing it for a few years. I find myself doing the same thing on occasion. You might hit 1500 words and want to put in another 500, but hang on to that, step away for a few hours, or wait until tomorrow if you're so disciplined. This simply helps the story from getting old while it's being created, and in my view and experience, keeps the energy high.
The last thing I can suggest, don't plot anything. You have it in your head, but don't write down a plot time line. I've done this plenty of times, and I noticed a difference in my writing behavior when I do so. It's not entirely poisonous, not like telling somebody the story, but it has an effect. If you can stay away from writing 'Act 1, Scene 1' before you begin the narration, it helps. Takes notes, if you have an idea for the story and are afraid you won't remember it later, please God put it down for your own sake, but don't write the abbreviated version of your story.