How do you reduce the weight of a helicopter?

Remove fuel and or the passengers. Yes, you can remove fuel by selecting engines that are more fuel efficient OR design rotor blades that produce more lift, thus allowing smaller engines requiring less fuel. Some helicopters are 2-engine. This has benefits of redundacy but the engines, gearbox and controls add more weight. The Transmission is a major part of the helicopter and many are made out of Magnesium alloy. During design, the engineer will consider building parts our of composite material which includes fiber-glass (as used in boats) or graphic-composite or even plastics. Seats are made with lighter material that is designed for shock attenuation in a crash. Landing gear can be made lighter using composite structure that is designed to crush during a crash.