These days I like to stick to one story at a time. That is hard. So many ideas whip around in my head while I am working on a piece. The funny thing is that they never materialize when I'm between stories. So, here is how I address this situation( I say situation and not problem because I think getting ideas is always a great thing and it doesn't matter when they come).
Years ago I used to keep a computer file of synopsises. But today that doesn't work because I find those ideas buzz in my ears like gnats and I eventually and start writing a first page of a story I will never finish. Here is what I today.
Now I go to my Write It Now Program and I start planning the second story. I mean meticulously planning it. I do this in my off hours when I am not working on my other story. Out of this plan comes an exhaustive outline. And I do a template for the story as well. Once all of this is done, I just leave it and go back to writing my current piece. Eventually I finish a draft of the story I am working on, and after a week's break, I go to the notes of my other story idea and I begin writing my first chapter based on the outline that I created days ago. Then the story, to use a shop worn cliche, practically writes sitself.
This strategy is responsible for my ever increasing story output. I don't know to me it's actually so important to see your ideas through.