OFAC tut blender IK Hints
From OpenFrag
Elbows and Knees IK chains
Since Blender 2.46 RC1 release there's quite nice tool helping with animating aforementioned joints, that is Pole Target for IK. Bacically it's second control bone for IK chain, to control which way the chain bends.
To use it you insert name of contol object in a editbox box of ik constraint, (right of main control bone editbox)
Very important thing is that it would be x-axis of bones in the chain that would be pointing towards pole bone, so you need to position all bones corectly in editmode, adjusting bone roll with "Axes" selected in skeleton view control. For elbow joint x-axis of upper arm bone should point backwards, pararel to global "y" axis. Back in pose mode, we now can lock X and Y axes for upper arm bone, so we now can stop worrying about if the elbow joint would bend right. It has simply no other option :)
I found that kind of setup far more intuitive than twin ik-chains i was using before, and much easier to control.
 front view |  side view |
