I had a small bit of board that I drew onto with pencil, then scanned digitally to sort out the colors and lighting situation in Photoshop; color studies are a good habit to get into before starting on finishes.
You can see that it's flipped horizontally. I'll flip images while still in process to check for errors.
Using a mirror does the same trick, if you hold your picture up to a mirror as you are working on it from time to time, errors in composition or proportion will jump out at you. Your eyes get used to seeing something a certain way and you can become blind to mistakes that are obvious when reversed.