[mgrassotti]

Used to recommend just paper, but these days my favorite is http://popapp.in - basically you sketch app screens on paper, snap photo of each page and then stitch them together. it's not going to help with process but is a great way to nail down an idea

[aladial]

Nice app, thanks for sharing. But that works for wireframing. If you're planning to give a different look and feel than the default iOS or Android theme I'm sure you use different tools.