Use a constant force spring(aka gas shocks) to balance some of the arm weight, put a potentiometer or encoder of some kind on the arm shaft, then tune it to move to a spot and hold position. I am a very big fan of gas springs/shocks, there are a fantastic way of supplying a persistent amount of force to a mechanism. In this case you would be using them to make your arm almost weightless and has the added bonus of acting like a motion damper to slow down any rapid motions.
http://www.mcmaster.com/#gas-springs/=1035oyj