Marcus and John:
The following thread is now (amazing to me) four years old. Having read it again I don't think I would change much.
http://forum.portraitartist.com/showthread.php?t=958
I don't think the approach differs much when you move from a four year old to a office holder. The exercise remains one of managing the light, and designing an artful composition.
I would like to say once again that every photo you take can be a study in composition and the design of light. These issues of design trump all other matters. Whether you paint from life or from photos you must
first arrange your subject, and design the light to fall upon them. The camera is a great tool to study these most important issues. In a single afternoon you can make many attempts which can be studied and critiqued by yourself and others. Make your attempts as complete as you can from border to border and as artful as you can. Be particularly mindful of the background.
Find a friend, or a bowl of fruit, and set out to make the perfect composition. Before you can paint it you must design it.
There are many good examples of corporate / political portraits to study in SOG.