@Sheldon: Oh, I don't have any guts in particular. I think what I rely on is discretion. If you don't particularly look like you're taking someone's picture, they usually don't think you are. Just holding the camera away from eye-level is enough, really.

The low camera angle in this picture comes from walking while holding the camera at waist-level. Of course framing can be difficult with this technique. Another method is to stand or crouch for awhile with the camera as if you are concentrating on something far away. Then as people approach or pass by, you can snap them. This doesn't work on people who are stationary, though.

Long lenses are also good for capturing from a distance, and wide-angle lenses work when the subject is close, but off-centre.

Personally, I don't mind having my picture taken in public, but I don't enjoy strangers sticking their cameras in my face without asking first. I wonder how others feel.