[GeeTwo]

Mostly, we stopped using 80-20, started using KISS. It's better to never build a 10 pound tertiary manipulator than to shave 10 pounds off your drive chassis or primary manipulator in the last few days before bagging.

As others have said, use materials (especially fasteners and adapters) with good strength-to-weight ratios.

Also, use parts that actually fit together with at most a single adapter. For a prototype, you can bolt two hubs together for a shaft adapter. On the real robot, use the right shaft from the start or spend the money for a COTS shaft coupler.

Develop a weight budget and weigh sub-assemblies regularly (every build session). Learn to recognize that you're going to be overweight early enough in the process to take corrective measures (parts replacement, redesign or reallocation) that preserve functionality.