I'm always trying to improve my photography. I read or subscribe to several blogs: Strobist, Joe McNally, Photofocus and Silber Studios just to list a few. In browsing some older posts on one of them I came across one with Charles Cramer on using a framing card to train yourself to compose the image better. Here it is. This technique was taught by Ansel Adams in his workshops. The video mentions how you can use it to see the image in 2D as opposed to the 3D that your eyes see. Also how squinting with one eye increases the contrast of the object which makes things "pop".
So I made one up and I have it with me even when I don't have my camera with me and I hold it up to various things where ever I am. It's an interesting exercise.