While you're waiting for the police to arrive, exchange information with the other driver. This includes names, addresses, and telephone numbers, as well as each driver's automobile insurance company and the appropriate policy number. In some places, it's a law that each driver must exchange this information before leaving the scene of the accident. If the other driver refuses to exchange information, just take down the number on his or her license plate, and the make and model of the car, and report this to the police.
You should also get the names, addresses, and telephone numbers of any witnesses to the accident. Statements from witnesses can prove valuable if the other driver (or the other driver's insurance company) challenges your version of the incident.
Note: For very minor accidents, drivers may decide to handle the situation themselves, without including police or insurance companies. If both can agree who was at fault, and make their own arrangements for repairs and payment, handling it this way can be easier and cheaper. Just be sure to get the agreement in writing and signed by both parties. (Even if you decide to go this route, however, it's a good idea to exchange names and phone numbers, just in case something unforeseen causes one of you to contact the authorities.)