People photography requires a special skill & it comes with practice. I know whenever two strangers meet it's difficult to crack the bottleneck of not being comfortable with each other unless you have some work. I  have found most of the people (at least in India) are eager to be get photographed. As I have written in the description of my many images on Flickr I try to have conversation  with them. I take their early shot knowing that they are not my best one & try to have communication with them. I try to realize them that they have no harm from me, my shooting & I am not at all  less than a friend. Winning the confidence of the subject is a key. After only that you will get nice expressions from them. In children photography also it is very important to be patient & giving the kids there time to talk. Kids are impatient & it's your turn to understand them, balance the situation  by being patient. Giving breaks in shooting helps them to relax. Experimenting with them adds their interest in the shooting. Giving some gifts like toffees & chocolates also helps. You have to talk with them as if you are a kid, that greatly makes them comfortable & more are the chance to get nice shots. Keeping the camera to the eye level of your subject really makes them comfortable as well as viewers of the image.

 In family portraits while shooting with 2-3 people you have to arrange them to portrait the relation,emotion along with affection. You have to comfy them for getting close enough to get nice compositions.