Our society has slowly moved to a place where people no longer smile or greet each other as they walk by on the street, everyone is preoccupied with their own situation. Now this shift in society behaviour can work towards a street photographer's benefit because it is much easier to take your photos unnoticed.
But if you are walking around quiet and sullenly taking your photos, it almost has a stalking appearance and that could put many people on guard while you are around. By smiling lots and saying hello to people as you walk around you become a lot less threatening. This will allow your subjects to relax while you are around with your camera, allowing you to get better photos.
One exercise I like to work on each day is everytime I am out taking photos I like to take the time to stop and talk to at least one stranger. By spending some time striking up conversations with strangers you will not only develop your social skills, but your confidence will improve dramatically.
This added confidence will allow you to continually be able to get closer to your subject so you can get the framing and composition you want. And you never know, while you are having your conversation you may just get the oppurtunity to take an excellent candid street portrait :)