i usually focus on their eyes, and while i am fumbling behind the camera i have them close their eyes
and relax ... when i am ready, sometimes i ask them to open their eyes, and eventually take the photograph.
if i am on location, i am often times a fly on the wall, and i use a fill flash with the lens stopped down to a bit ...
or i do the same technique as i mentioned first ... i like using a graflex slr to make portraits as well,
like a slr 35mm you can see what is going on the instant before the shutter is tripped.