At first glance, I'd suggest a worm gear setup - it requires no motor power to stay in one place. You could also implement a position PID controller for the motor, although this will draw lots of power (not necessarily a good thing).
A way you could reduce the force would be to use a constant-force spring to reduce the load (acting in the opposite direction on the arm). This allows your arm to be, relative to the motor, a tiny fraction of its weight, and easy to keep in position.