For the last ten years I have always pre-booked through a broker called : carjet.com.They will send you a confirmation telling you which company desk to go to on arrival.They include excess on the insurance and up to three drivers.At the desk you need to produce the confirmation,Drivers licence,cc, passport. Some companies will offer an extra insurance for 10-15 Euros to cover Tyres,windscreen,underside of car.... Having paid out once 120 Euros to replace a burst tyre,I think it's worth it.