Requirements, construction methods, material selections, weight allocations per component/subsystem, a lot of CADD modeling with weights included in the materials, the list goes on.
Plywood is a great building material, especially 1/2" Baltic Birch.
0.063" and 0.090" sheet metal with 1/8" steel rivets will do a lot of the super structure for many robot designs at minimal weight (<10 pounds on many occasions).
Set weight allocations for drive, chassis, command and control (sensors), electrical and wiring, mechanical arm and end effectors/shooters, etc. One thing you can do to help for the future is to track your robot weight every year by weighing before you build, or taking apart old robots. Learn how much weight 1/4-20 fasteners add and realize that a #6 or #8 screw can handle the load (yes, you can hang a robot in shear with one #6 screw, but don't try this
).
Another suggestion is to track your weight on a weekly basis to see if your targets/guesses are correct or on track. This allows you time to go back and redesign or make compromises in the trade space.
I hope that helps!