I use paper and pencil for my mockups. They've got the unique property of not making you think about details, but allowing you to put them in at any level. You don't need any skill either. It works even if your drawings consist of lopsided rectangles.
> They've got the unique property of not making you think about details
Paper and pen is good too, but it makes me think about details like:
* Whether a line is straight
* Whether my writing is legible
* Whether my writing fits in the container
etc. I don't have to think about these when using a drawing app. Also I can send my render to an XML slicing service. On the other hand, connections are much easier to draw on paper.
