[mindcrime]

I wouldn't use UML for high level / rough idea stuff, although I am a fan of UML to some degree. I don't use it as much as I used to, however.

That said, I go the old fashioned way for sketching out ideas: an artists sketchpad, some drafting pencils, a box of colored pencils, a straightedge, etc. I mostly just draw boxes, lines and arrows. Interestingly enough, this actually can (at times) be a sort of "pseudo-UML".