I practiced a lot this weekend with Eric Kim and improved a little with the following approach:
1. Shoot from street curb back into buildings.
2. For this shot you can increase your odds if you as the shooter are fixed, not moving.
3. Hold camera chest high, not at eye level until the split second to make the shot.
4. Manual focus with pre set distance helped make a quick shot as this is all about timing.
5. Good light helps so you can keep the shutter at 1/1000 and strong depth of field, F8 ish. I personally shoot at ISO 3200 for everything as I dig the grain and vibe when it happens.
6. Pick an anchor image/person in the back ground. It helps a lot if they are not moving.
7. Put the camera focus on this image/person.
8. Wait for new, moving people to enter frame. Three or five total is usually better that two or four.
9. Try to get strong bookends for the left and right frame lines. This could also be a tree or sign, etc.
10. These shots happen quickly so some pre visualization helps. Colors, textures, shadows, etc can all be worked into these shots.
11. I found it helped to literally step in 2-3 feet as the scene came together. So standing on the curb and then smoothly stepping forward as I raise the camera and fire the shutter all in one move. Most people were unaware I took the shoot, no adverse reactions.
12. These layered shots become even more interesting using heavy shadows and slices of light. Dialing down the exposure compensation is a good move here.
13. I also want to try this with flash as it helps freeze the moment. As I am coming in at 90 degrees, it is not as confrontational as a Bruce Gilden frontal.
14. I shoot these with a 16mm on 2/3 sensor so best to be very squared up to the back focus subject. But a slight camera rotation puts some edge in the image.
15. A fun technique is decapitation, where you purposefully cut off the head of one of the passing subjects. It can make the scene more dynamic.