I have referenced Kirk Tuck and The Visual Science Lab a few times. The common sense approach that he takes to issues just speaks to me. Below is one of his posts about how he approaches strangers for portraits. I am composing this on an iPad: so far, I cannot see how to make this URL a link. Please copy and paste it into your browser. Thanks.
http://visualsciencelab.blogspot.com/2013/09/say-hello-look-people-in-eye-and.html
Enjoy.
Bob