This blog post is one out of several blog posts on improving your user interface sketching techniques. You might want to read the first three posts: Drawing corners and boxes, Drop Shadow, and Use a thick pen.
Get your arm off the paper
Resting your arm on the paper while sketching locks your lines into arcs, why trying to draw straight lines will only feel awkward and come out wrong.
Instead you should focus on drawing from the shoulder and not your lower arm. This will allow you to get a much better feel of both the paper and what you’re drawing.
