Photograph the accident scene.
Measure the length of any skid marks or gouge marks in the roadway.
Photograph the interiors and exteriors of all vehicles involved in the accident. (If you believe the injuries to you or your passengers were caused by a defect in your vehicle, such as the failure of an air bag to deploy or the collapse of your seat back, it is crucial that your vehicle not be repaired or destroyed until an automotive design expert has inspected it.)
Get the name, address and phone number of anyone who witnessed the accident or who arrived at the scene shortly after.