Compare your arm movement and elbow positions across the throw to this: http://www.youtube.com/watch?v=XHSZyYAVPbs
Notice that he reaches straight back from the target (gets his shoulders fully perpendicular to the target) and pulls so that his elbow leads straight to the target. While the elbow is getting forward his shoulders and hips are quiet, only unloading once he starts to enter the release zone of the throw.
You reach back by moving the disc in an arc around your body (needs to be a straight back reach, not an arc), You then pull through in an arc around your body, unloading your arm, shoulders, and hips all at the same time. (too much hip movement, too early shoulder movement).