I often look for new composition ideas by cropping an existing piece of artwork in Photoshop. This can also be done using a viewfinder made with two L-shaped pieces of paper or mat board, but you have the option of enlarging the cropped area if done digitally. The above is an encaustic painting on paper, 9"x9", using wax (encaustic), collage, and India ink. I scanned the piece, copied it several times, and made the following cropped images:
The next step will be to either do sketches from these images, or jump right in and start some new pieces of art using these as inspiration. In any case, I will print them out and glue them into my sketchbook for reference. It gives me a new perspective on existing work, and fresh ideas for new pieces.