I'm not the type that gets stuck for ideas. In general I have an over abundance of them. However, it often takes many years for these ideas to turn into a book. I may be creating worlds, building life stories of my characters, manipulating their relationships, but even when I can see and feel all this I have to decide what the story is about.
Great stories can usually be summed up in a couple of lines. Pride and Prejudice is exactly what it says on the cover - the story of two young lovers who have to overcome pride and prejudice. A Christmas Carol is the tale of how an old miserly man who hates Christmas is given the chance to reflect on his life and discovers a new found love of life and humanity. Rebecca is the story of how the influence and circumstances of a dead first wife almost destroys (the never named) heroine's marriage.
Each of these stories is rich in sub-text, weaves and interweaves plot, but never loses sight of what it is the core sense of the story.
One of the difficult parts of being a writer is not digressing. It might be hard to write 50,000 words, but it's much harder _not_ to write 100,000 to tell the same story. For me, it can take years to figure out what a novel or play is about, but until I have that two or three line precis I know I mustn't unleash the torrent of story I have conceived in my head. If I don't wait I end up with endless rewrites, a plot so huge it would take ten tomes to do it justice and eventually having to lose much of my hard work to create a thinner, sleeker piece. When I manage to find the patience to wait I produce my very best work and the story does that magical thing of writing itself.
A Death in the Family was one of those magical stories. Euphemia, my heroine, is loosely based on my great grandmother and as a character has been with me for most of my life. But not everything takes that long to gestate. My short play 'Breakfast After Dark' (on at the Traverse, Edinburgh on Nov 15 as part of '10') took weeks to think about, but only as long as it took to type to write. (And the director was very pleased with it.)
In essence - don't write too soon. Carry your story and let it form before trying to put anything on the page. If you're attempting to earn your living by writing then the nice idea of writing only when the muse strikes is not profitable. There will always be times when it is knowing your craft rather than divine inspiration that will get you through. Personally, I carry many stories in my head. It makes me absent-minded and distracted. I've been known to end a telephone conversation by apologizing that I need to be in 1910 now. But the brain is a wonderful thing. I can carry many stories at once. My mind works on plots for while I sleep, eat and go about my daily life. Feed your mind with experience, make small notes and stay aware of all the stories you have and hopefully one day your stories will feed you. If nothing else being so distracted gives one an excuse to be lax with the housework.