Hi-fibre writer's diet

I don't claim to be any great shakes in the writing department. I like to tell stories and I enjoy writing. Most of my writing is (pseudo-)confessional and the disclaimer on the right (under my avatar) should tell you all to need to know about the truth:invention ratio I employ.

But right now I'm working with my brother on a short story. We've spent a couple of weeks bouncing ideas off each other and then he wrote the first draft. I edited and returned. He started working on it and hit a wall. I said I'd take over. I've been writing for years, both professionally and in other contexts. He hasn't, he's a recovering artist and creative writing is new to him.

So the ball is in my court. But...when it comes to any writing we know it's hard to get started, hard to get focused. Once you're going it's usally fine, but getting going is tough. And when you're re-writing someone else's work, even if it is mostly collaboratively, all of it is 'getting going'. What does this paragraph need? What does this thread/arc need?

I am in the depths of imaginative constipation right now. I spent a week writing in my head, developing the story arc (as much as you can with a very limited word count). I developed the character a little, added some details, worked on a little depth.

...and now I'm in front of my computer none of the ideas will come out. I'm so full up with old ideas, they've formed a stiff and tight blockage and now I'm all bunged up. I need a mental laxative that keeps me cogent and my fingers working... suggestions?