There's 3 basic methods for keeping an actuator stationary against a constant load (ie. gravity):
1. Counterweight/balance. Balance the force you don't like with an opposing one from springs, weights, etc. For a pivoted arm, you could put a spring or weight on the opposite side of the pivot.
2. Anti-backdrive transmission. Certain styles of transmissions are good at transmitting torque from the input to the output and REALLY bad at transmitting torque from the output to the input. Think of them as one-way streets for torque. So the motor on the input can move the arm, but the arm isn't going to move the motor, and thus the arm isn't going to move on its own. Worm-drives are good at this. Many drills have a system like this that locks the drill chuck when you try to turn it, letting you loosen a keyless chuck. The window lift motors (I'm nearly certain) also have this style of transmission.
3. Feedback control. You put a sensor (typically a potentiometer) on the joint you want to control. Then you have the CRIO compare the position of the sensor with the position you command the arm to be in, and power the motor accordingly. This can actually work in concert with the previous two methods to sharply fine-tune the position you want the actuator to hold.
Your ultimate solution is going to be up to you, obviously. If you're looking for anti-backdrive transmission besides the window lift motor, the
Nothing But Dewalts white-paper tells you how to stick a Fisher-Price motor into a Dewalt drill, getting you both a nice reduction, selectable speeds, and an anti-backdrive mechanism. It's a little dated at this point, but I think most of it is still valid.