In fact, of all instructional design activities, I have always found storyboarding the most challenging.
One work around I have found for this problem is to take screen grabs of ideas and layouts that I find interesting and save them for future reference. So when I'm stuck , I browse through this library and get some fresh ideas.
I'm sharing one of my most recent layout repositories here. In this document, I have classified the layouts into areas based on their possible uses for convenience. But you can pretty much use the layouts any which way you want.
The repository is at http://www.scribd.com/doc/63483353