You can also use your fingers to create a viewfinder. Hold out your hands, with their backs facing you. Straighten your thumbs and forefingers, then twist your one hand around so you've an L facing in the opposite direction to the one created by your left hand. Now put your two hands together, and you've an instant viewfinder.
The disadvantage if, of course, that you can't paint or sketch while you're doing this!
