I got an email today from a young writer dealing with a common problem among all writers, but especially young ones---great ideas that don't go anywhere once you start working on them. This is something that happened to me all of the time when I first started writing. I'd come up with an idea that near blew my mind and then I'd dive right in. Sometimes I'd write up to one hundred pages before I realized I had no idea what I was trying to say with it.
I don't dive in so much as wade the waters these days. I still have those dynamite ideas, but I take my time to make sure it has a center that's going to hold before I commit to writing it. A good story, like a sturdy house, needs a foundation of solid building blocks. Always make sure the material you're working with is going to withstand the big bad wolf when he comes to huff and puff and blow your whole story to shit.