I can only add one very useful thing to do before anything else; Make pictures from every angle, and from both vehicles involved.
Specially if they carry 153 water-bottles on a scooter, 5 people on a motorbike, or if the other parties bicycle is laying on the wrong side of the road (before they sneak it to the other side before the police arrives...).
It saved me several times from nasty accusations, heck, I even got paid sometimes :)