I want to share the work of somebody from another forum. I thought his solution to vibration problems in multi-rotors is pretty novel, a good implementation, and quite effective. It appears that not only does damping the motors protect the frame (and thus the flight controller) from vibrations, it actually reduces vibrations in each motor since a vibe in one motor does not upset all the other motors. You can see this from the sound produced.
Just to make it more clear what he's done, the blue part is rigidly attached to the arm. the motor is bolted to the upper mount black mount, which is connected to the lower black mount via a shaft in the middle. The lower black mount pushes upwards on the blue pieces via 4 rubber vibration dampers. I believe it was done this way because those dampers are intended for compression loading, not tension.
And the result:
This comes from this thread: