There are a lot of drills you can do to improve your arm stroke as you swim. One of the easiest is a single arm pull. For this drill, start with both arms extended in front of your head. Perform your scissor kick, and slowly pull one of your arms back. Concentrate on rolling your trunk with the motion while maintaining your balance in the water. Pause with your hand on your hip before bringing the same arm forward. Pause again with your arm extended in front of you before you begin pulling back.
A variation on the single arm pull is the double arm pull. The motion is the same, but instead of doing one arm for a set distance, you alternate arms. You still need to keep the motion slow, concentrating on hand placement and trunk rotation. You can also do this drill with a pull buoy -- a flotation device that you pinch between your thighs so you don't have to worry about kicking.
The most efficient swimmers cover the maximum distance with each stroke. Try doing a single or double arm pull drill for one lap. Count each stroke you do. On your next lap, try to decrease the number of strokes you take without decreasing your time on each lap.
Another good drill is the fingertip drag drill. With this drill, you're still going to swim slowly and deliberately. As you bring your arm forward, drag your fingertips lightly in the water. This drill will help you get the proper elbow position for the recovery phase of the stroke, which in turn helps you learn the proper position for your hand's entry into the water.
Other freestyle arm drills involve playing with the rhythm of the stroke. Try doing two strokes on each arm before switching. Changing up how you do the stroke will not only keep your muscles engaged, but it will also force you to concentrate on your stroke, rather than just repetitively going through the motions.