Hardening Motors
The shocks and forces that combat robots endure during matches are far higher than in almost any other application. Just as you can pad the internals of your robot with foam, or have well-planned strain relief in your wiring, you can harden motors and make them more robust.
Reinforcing motor magnets
With brushless motors, a common failure is the cracking of the motor's magnets on impact. Robert Cowan has a great video on how to use glass microspheres and epoxy to strengthen the magnets.
Links are provided by Robert.
Replacing motor shafts
It's not uncommon to reinforce shafts with an additional bearing or pillow block to resist side impacts on the shaft (radial impact). However, many builders have found that the design of the motor itself is a weakness to be overcome. Here's an example: