The flight was being pushed back by a tug when it contacted a lavatory service truck, which had pulled up at the rear of the airplane to service the lavatories. The airline uses one person pushback operations at this station for B737’s. Under the procedure, the ground handler will walk around the airplane just before the pushback to ensure that the airplane’s path is clear. Once the flight crew gives the clearance, the pushback commences without the aid of wing walkers or other safety observers. The handler assigned to this flight did his walk around after the jetway bridge had been moved back. Once he was cleared by the flight crew he leaned over the tow bar for one last look and then mounted the tractor to start the push. He did not see the lavatory service truck at any time and the lavatory driver did not verbally communicate his presence or intentions. The flight crew turned on the rotating beacon just before the aircraft started to move. During the push the handler felt resistance and stopped just as the lavatory service driver ran to the front of the airplane shouting for him to stop. Three calls had been made for lavatory service on this airplane, the last one 10 minutes prior to departure. The lavatory service driver originally assigned to handle this flight was busy. Another lavatory service driver heard the service request on the radio and responded to the airplane, arriving about 7 minutes prior to departure. When he arrived, the rotating beacon was not on, which signified to him that the pushback was not imminent. He parked his service truck under the tail of the airplane and proceeded to service the aft lavatory just as the airplane began to move and it contacted his truck. The airline’s ramp operations manual states, ‘Before approaching aircraft to perform last-minute servicing, at or near departure time, always establish positive verbal communication with the pushback tractor driver.’ Probable Cause: The lavatory service driver’s failure to follow established company procedures and directives. A factor in the accident was the airline’s use of a one person pushback procedure.