- Select bones you want to mirror.
- Shift+D to duplicate.
- RMB or Esc to drop selection so you wont move by accident.
- Hit "." to set mirror center to 3D cursor (if 3D cursor is not on origo, hit Shift+C to reset).
- Scale nevative X, so type S-1X and enter.
- Go to Armature menu (mirrored bones still selected) and Flip names.
- Select non mirrored side of bones and Ctrl+R to mirror correct roll.
- RMB and drop tool.
Tip: Remember to turn on X-Axis Mirror from Armature Options under Armature Tools (T on 3D view).