Nikon continuous-servo focus mode
When setting up a shot like this, the first thing I do is switch my focus mode. I typically shoot in AF-S (auto focus single) mode when doing portraits but every now and again I like to have the subject walk to catch some basic action. I switch to AF-CI (Continuous-servo) mode so that the focus follows the subject as they walk toward me. The key here though is to set your fstop at a high enough number so you allow for a little more depth of field, because the auto focus is constantly working in and out and you could miss focus a bit if the aperture is set too large. I try to stay around f4-f5 for this. My camera setting for this shot were ISO400,1/125 sec at f/4.0,with the 85mm f/1.8 Nikkor lens. Some nice illustrations of the different focus modes on Nikon DSLR cameras can be found here.