Paul Mobley does mostly portraits, a lot of musicians.  What he says about photographing people you see on the street applies to all kinds of photos of people.

I’ve worked with him, and it is true that he never stops talking to people.  He’ll go into a situation and, with humor, take control and tell people what to do. And they LOVE it.  People want the pictures you take to be good, and if they trust that you’ll make them look interesting, they’ll do what you want.

http://www.mobleyshoots.com/2013/06/youve-got-a-great-face-now-what.html

http://www.mobleyshoots.com/2011/02/try-to-make-a-connection.html

Take a look at his photos, too:

http://paulmobleystudio.com/