First thing is first, make sure that everyone in your vehicle and the other vehicle is alright.
It helps to have pen and paper and a digital camera ready for the next part.
Take note of the time and date, location of the accident, environmental conditions, direction of travel by you and the other party, makes and models of both vehicles. Be as detailed as possible to the point that it would seem a complete idiot will be reading the data. (sometimes not too far from the truth)
Also get the license information of the other guy (name, address, license number, birthday) include yours as well.
Take a lot of photos of the accident scene and try to include the license plate numbers of the vehicles whenever possible. If you can print them on the spot all the more the better as it will hopefully save you from the horrendous charge the crocs (read: cops) will ask from you to get an "official photograph" of the vehicles (new racket, this one.) using a cheap ass digicam or a traditional film cam.
Write a summary of what happened and be as detailed as possible. And both of you must sign it.
This will help filing the report if necessary since the details will still be fresh in your minds and to make sure no one denies anything when the filing takes place. If you will not file with the crocs, er, cops, these details will still help you file the affidavits to claim with your insurance.
I guess this is it. If I have left anything out, please feel free to add
Hope this helps.