What works best for me when shooting natural light portraits is to place the subject in an area of open shade, trying to find a place where they are surrounded on at least three sides.
What is needed almost more than anything is to find a location where your subject’s face is brightly lit, and the falloff of light starting about the ears or back of the head is pretty abrupt. What works best for me when shooting natural light portraits is to place the subject in an area of open shade, trying to find a place where they are surrounded on at least three sides. Place your subject in the shade and as close to the line of light as possible. (see diagram)  Bonus point are given if you can find a place where your subject is in the shade, you are standing in the sunlight, and there is something large and reflective immediately behind you like a building or a light colored fence.