Watch the Right Forearm online guitar lesson by Bruce Arnold from Guitar Physiology Survival Guide
As previously mentioned the forearm should be the initiator of movement for your pick. This movement is the same one you use to drink a glass of water or screw in a screw with a screwdriver. This technique takes a while to develop but by using this natural movement you can play all day and not have any issues with overuse. If you are a beginner this movement will come faster than if you are switching from playing with your wrist. Something that will help both the beginner and more experienced guitarist is a "scratching the string" exercise. This involves placing the neck of the guitar over your right shoulder (like a double bass or a cello) and picking the strings with the flat side of the pick. The idea here is to fool your mind into thinking it's not playing the guitar. This will give you the right feeling about this technique. Once the movement is stronger and more ingrained it will be easier for you to apply it when you play the guitar in its regular position. One thing to watch out for is to keep your fingers relaxed especially the ring and little finger. This exercise is something you can do as you are watching television or reading a book. Once you get the movement correct it's a matter of repetition until your mind and forearm connect rhythmically. Most common problem is not moving from your forearm but using your wrist so watch out for that. Start with large movements to get used to playing from your forearm. Over time limit your movement without tensing up. When you get to the point of feeling like your arm is shimmering you are very close to mastering this part of the preliminary development.