[lofty rhino]

I find that carrying around a pocket journal suffices for the most part--usually use it for exactness of detail.

I would use something like an iphone app, but my fingers are dumb.

Just as an aside, I would warn against doing *substantial* amounts of composition on-the-go. When writing something like a whole scene, I'd recommend finding your own space where you can keep your mind organized, and clear of too many zany thoughts.

Creativity is great, but when it is coupled with intent & focus, it becomes something unstoppable.