Mirroring an object

I’m working on a model of a humanoid, and was wondering how I would go about mirroring an arm. I have all the bends and joins worked out for the first arm, and would hate to redo it all for the other arm.

I tried duplicating the arm, but couldn’t find out how to mirror it after that.

Sellect the mesh you want to mirror and press M key in the Edit mode.

Simpler: use a Mirror modifier.
http://mediawiki.blender.org/index.php/BSoD/Introduction_to_Character_Animation/Setting_up_the_mesh#Mirror_the_plane

Make sure your object center is where it needs to be, or you’ll get overlapping results. Once you’ve finished the entire (half) model, just apply the modification in Object mode.