One way to photograph people on the street without alerting them to your intentions and thus making them uptight, self-conscious, angry/violent is to simple set your camera to take a timed shot. Set time as short as possible – 2 seconds or less is best. Release the shutter whilst facing away from your subject and then casually turn around and allow the camera to do all the work – photography sans hands!
In order to not alert your subject look anywhere other than in their direction. Be cool – read a book, text a loved one or simply place hands in pockets. Do not touch the camera or your prey will flee/pounce!
This technique works especially well if you are able to set your camera to multiple timed exposure!
I was standing fairly close to the two gentlemen featured in the below image and I’m sure that if I had raised my camera to compose the shot I would have alerted them and the moment would have been lost.
Taken with a Nikon D5000 + 18-55mm.