Joe Tucker has a drill that works very well to help fix or change a stroke. In a nut shell it goes like this. Place the cue ball on the spot and an object ball on the head string so that it is perfectly straight into a corner pocket. You are going to shoot this shot five different ways. The first way is to draw the cue ball back to the foot rail. The second is to draw the cue ball back to about the side pockets. The third way is a stop shot. The fourth is a slight force follow. The last way is to follow the object ball into the corner pocket. Shoot each on of these with considerable speed so that any flaws in your stroke are exaggerated. Use this drill to practice not dropping your elbow. You will know you are not dropping it if your cue tip ends up touching the felt within a few inches of the cue ball. A video camera will help a lot in learning what it feels like to not drop your elbow.