Well, I have in my portfolio many portraits. They are travel related rather than formal; however, I have developed a habit I would like to share.  I keep a little pocket notebook and pen with me all the time. When I notice some place which just seems like an especially interesting background for a portrait, I write down the location & details. I find it also helpful to note the light direction, and what it is I find special about the place or why I have noted it.