How can I mirror bone rotations? I have a rigged character. And like most character models, I have modeled it in the "T-pose". Now I want to create an idle animation, so the arms need to be resting by the character's side. I selected both the upper arms and tried to rotate the arms simultaneously. This causes the one arm to lift and the other one to rest 

Like so:
 

I was wondering if there's a way to mirror the rotation? Most of the material I could find regarding this simply "Flips" the armature.

Thanks in advance.
R to set rotation mode

Ctrl+M to set mirror mode

In this case X key twice

You'll see two red lines cross over your character in front view.