One of the weak spots of the current 3DR frames is that the aluminum walls of the arms are very thin. The motors mount directly to the arm and in the event of a "landing anomaly" the aluminum wall is easily deformed by forces on the motor.
My solution was to construct "backing plates" for the motors. The plates fit inside the arms and slightly longer screws are used to attach the motors through the plates. The plates are simple 1/16" aluminum sheet drilled to match the 3DR mounting holes. The small indentation you see in between them in the picture below is for clearance of the motor axle, although it is probably not necessary as I don't think the axle protrudes further than the thickness of the arm wall.
This addition should improve the frame robustness somewhat, hopefully without merely identifying the next weakest link.