If I have a bug bear about Street Photographers, it’s this, it’s their inability to sense a state of unease in their subject. This in turn can lead to a photograph being destroyed through a blink of an eye, or a turn of a head. Never take a photograph of someone who turns their head, is my advice. They simply don’t want you there, and so you have no right to invade their space.
The other side of portrait Street Photography is catching people when they are unaware of your presence. This is where all the greats started, and finished, their careers with a camera. This is where you should begin. If you can’t get someone to sit for you, then you sit and wait for them. Softly, softly, catchy monkey.
The Street Photographer's Guide 