Arm swings when turning, how to brace it?

[HarveyAce]

Or arm is made of a single four foot aluminum c-channel with a fabricated 8020 bracket at the top to hold our window motors for the shoulder joint. the next stage is another three foot c-channel with the gripper on the end. we noticed in the little practice time we had that the channel tends to twist when we turn. Does anyone know of a way to brace the channel so that it won't twist? there are some pictures on the portal of our robot from last monday (team 3329) if that helps anyone. Thanks!