I "shoot from the hip" pretty extensively using my 5DII.
I leave it on Program-Auto most of the time, sometimes Tv, so you can look like you are just gesticulating or waving your arms (with the camera, incidentally) about while taking exposures. Make sure you activate all the autofocus points, and take several shots for each scene. Move the camera slightly and retrigger the AF between each shot, this increases the likelyhood of getting what you want in focus.
You could stop down, but I really like having a shallower depth of field for street photography.
Obviously, there can be some issues if you are in a situation where you have multiple things in the frame, with significant differences in distance from the camera, but for me, at least, it doesn't seem to happen that often.
Also, the distance scale (on most of the good Canon lenses, anyways), is accurate enough that if you do have a challenging composition, you can guestimate the distance to the subject and dither the focus back and forth while taking a few shots, to increase your likelyhood of getting a shot in focus. With the speed of modern cameras, this can all be done in a couple of seconds, so moving scenes can still be shot pretty effectively.
Mostly, experiment, and always take a couple of exposures, rather then just one.