Armature symmetry mirror

Before you mirror, make sure you have renamed all bones like "upperArm_L" so there is _L or _R suffix.

1. Select bones you want to mirror.
2. Shift+D to duplicate.
3. RMB or Esc to drop selection so you wont move by accident.
4. Hit "." to set mirror center to 3D cursor (if 3D cursor is not on origo, hit Shift+C to reset).
5. Scale nevative X, so type S-1X and enter.
6. Go to Armature menu (mirrored bones still selected) and Flip names.
7. Select non mirrored side of bones and Ctrl+R to mirror correct roll.
8. RMB and drop tool.

Tip: Remember to turn on X-Axis Mirror from Armature Options under Armature Tools (T on 3D view).