I'm only a First Responder, so LayinLo outranks me by a long shot. My small contribution:
Don't forget the ABCs
open Airway? (you may have to open it by using a head tilt/chin lift . . . be careful to move the head/neck only as much as you have to)
Breathing? (you may have to give CPR)
Circulation? (again . . the CPR)
Also, try to have someone (if there's enough help around) in charge of spinal immobilization. That person's job is to hold the head steady (assuming it doesn't have to be moved due to life-threatening injuries).
After a 4-wheeler accident a lady had around the corner a couple of years ago, I sat on my haunches (ask your mom . . she knows what that means
), holding spinal immobilization for what must have been 15 minutes or more. I remember watching mosquitoes land on me . . . bite me . . . fill up . . and fly away fat, dumb and happy. There was NO WAY I was going to let go of that lady's neck until I had a collar, though!
Lastly, maintain a safe scene. The very same traffic that injured the victim can take everyone on the scene in one fell swoop. Setting up some sort of barrier (automobiles work nicely) can protect you some. The main thing is not to let the other responders lose their sense of scene safety and start walking into traffic, unaware of the dangers due to the gravity of the situation they are dealing with.
Oh yeah . . . ride safe so you're the responder and not the victim.