Use a thick Sharpie
Sketching out a user interface is not about details but about concepts. It is about sketching – not drawing. When you draw, you tend to worry too much about making the details look good. This is why you might want to choose to utilize a medium or broad point permanent marker (Like a Sharpie pen or similar) to sketch your interface, instead of a thin point one – or a bullet point pen. So sketch big and focus in on loose form rather than tight line accuracy.
This is also why many find sketching interfaces on a whiteboard especially useful. Being forced not to draw details constraints your thought process into focusing on the concept and ignoring the details.
Some people prefer using crayons over Sharpies as Sharpies often bleed through the paper.
The above illustration illustrates how the sketcher tends to get into details when drawing with a thin marker. The thin sketch is both more correct and more appealing, but at the same time feedback on the thin sketch revolved more around details than the concept. The thick sketch was drawn about 10 times faster, and has its focus more on the concept. With the thick sketch, the sketch could be redrawn and iterated 10 times faster than with the thin sketch. It’s all about rapid prototyping and instant feedback!