Always carry a camera with you, even if it is your smartphone camera or a point and shoot camera. Better yet a compact camera with either a sort of normal lens, 35mm, 40mm or 50mm lens.
Also, try to anticipate what is going to happen next. Visualize how the elements in your frame will come together. The streets are an ever changing stage with split-second plays; if you keep your eyes wide open and the finger on the shutter release, you are able to capture some of the decisive moments; don’t you agree?