Beef I'm right there with you! I like to think I have an expansive imagination. I have ideas for 3 different book series, and two of the series I have ideas for multiple sequels. The problem is when I go to sit down I just don't feel like typing. I'd rather hook some device up to my head through a USB port and have the computer translate the novel from my brain. It doesn't help that I have a one year old who just doesn't think writing a novel is worth daddys time.
It definately helps me to plan the book out. Have you checked out Jim Butchers blog? It has his own set up with how to create a story idea and get it started. I agree with Lano that once you have your overall plot, write a short paragraph about it. I think Jim said try to do it in one sentence, but if you can't project your plot in less than a paragraph then it's too polluted. It has to be simple enough that your mind doesn't find it impossible to write from start to finish.
If I'm not typing my story then I at least always keep my mind on it, and come up with new ideas and try to fill in chapter gaps.
You should also figure out how detailed you want your first draft to be. My friend is a sucker for details of the scene, place, and character, so he adds that all in right from the get go. I personally get bored with the details, so I choose to skip them so I can at least get my story on paper. Once I finish my first draft I will go back and insert details of buildings and scenes. I call my first draft my story skeleton. You could also choose to skip any subplots you have, and just punch out the main story first and foremost.
Lastly I would say don’t let yourself get stuck by writers block. If you are typing out a scene and get completely stuck with where to go or how to describe it, then just make a note of it, highlight it, and move on. I personally have trouble skipping entire chapters, but if I’m having an issue with naming something or describing something important I can move over it just fine.
Some of this may work for you, some may not. This is just steps I take when I’m trying to write. I wish you lots of luck!
PS – You could always find a friend who loves to write and just double team the story out. There are times I wouldn’t mind this option but the only friend I have who is a writing fan is working on his own projects at the moment.