A classic way to perform this task in robotics is a PID controller http://en.wikipedia.org/wiki/PID_controller
Basically, the power commands sent to a motor is proportional to the angle error.
In the beta, we have provided a "DriveJoint" that simulates a motor (a HingeJoint with a TargetVelocity field).
Unfortunately, you can't read the current "angle" value on this joint.
In the release, we have removed the DriveJoint and added a MotorizedHingeJoint and a MotorizedSliderJoint where you can both set a target velocity and read the current joint position.
You can still use PID controllers with the beta but you have to write some code to calculate the angle between the two actors moved by the joint (by using their world matrix).