My thought is to take photographs of the progress of the piece at the end of each 'session' that I work. It helps me to see where I went wrong with a piece or where I could improve it. Sometimes, it lets me digitally enhance a part of the photo to show me what it would look like if I did that in real life.
Usually, I start with some reference picture or rough draft idea. After I loosely draw it out and figure out placement and composition, I try to remember to start on the background. Sometimes I get excited and jump right into the portrait. I try to stay 'kind of' close to the original source to get close to the likeness of the character. I don't really like to make my work look photo-real. I like to leave some brush strokes and expressive marks.