Something I have noticed lately is that you can become almost invisible by being very visible: instead of a small concealed camera, I have used a long lens (hood included), on a large tripod; I even wear a red vest, very visible from a distance. I guess that people somehow assume that you are doing what you are supposed to be doing, whatever that is, and stop noticing you. Obviously, you have to carefully select the background first, and try not to follow people with your camera.