In previous projects, aswell as using Concept Art I've also used Storyboards, but only in a limited sense. The last time I did create a storyboard was for an opening to a movie, what was notible was a huge jump in quality between previous efforts. So I feel I'm on the way to being fairly competent storyboard artist.
When looking at professional storyboards it seems that the type of thought process that goes into creating a storyboard is similar to that of a comic, lining up a frame of where camera angles will be. Ultimately showing what should appear on camera.
Another thing that should be noted is that storyboards can have varying levels of complexity in their drawings, but the most important thing is that the images show what should happen on screen clearly. More often than not storyboards are completed in pencils and tones, sometimes with Markers. What's important to remeber is that storyboards often show the action in the scene where as concept art's purpose is to show the detail and design of the items in the scene.
As with concept art as I research more about storyboards I'm sure more inforamtion about the artform will reveal itself along with tricks and techniques but the general rules seem to be. Basically it seems that a good storyboard contains Good composition and is easily readible as an action scene.