A few years ago I started using the "Practice Pro" pocket reducers. This creates a pocket on my table that is 3.5" wide. If you touch the sides at all, the ball will bounce out. Needless to say, this requires a damn straight stroke. What I have learned over the years is that accuracy is about 90% stance related. Aiming is a skill that most humans seem to bring to near perfection relatively quickly. At that point, making the cue go straight is the obstacle. Now the elbow works in such a way that if you can swing only your forearm, its pretty much going to go straight. So the final obstacle is orienting your body so that the swing of your arm is exactly on the line of the shot. This is the part that many people do not practice at all.
I have a particular method that works quite well for me and the few people I have tought it to. First, I stand behind the line of the shot, facing the shot as squarely as possible. Ideally, this means that the center of my head will be right on the line of the shot. By "line of the shot", I mean the line that the cueball will be travelling on. Rememeber-all shots are straight. You only ever shoot the cueball straight (excluding masse shots). So here is the order of events from this position:
1) Stand at the right distance from the cueball. This will vary for everyone. For me, the distance is about the length of my cue. I am just over 6'2", so this is probably wrong for most people. I stand with my feet about shoulder width apart.
2) Place my back foot (the right for me) on the line of the shot. For me, I place my heel on the line. Different people might prefer a different part of the foot. Maybe the toe, maybe the instep, etc. You will have to experiment and find the right part for you. I am not stepping forward with this foot. I am simply turning it about 45 degrees to the line of the shot and placing it on the line.
3) Next, I step *straight* forward with my front foot. The amount I step forward is enough so that I feel balanced. This results in a situation where my body is roughly 45 degrees to the shot, maybe even a little more.
4) At this point, I will be holding my cue verticle. My looking "through" my cue, I can see the line of the shot. I am holding the cue where I would normally, and my bridge is made around the cue in the place where that would normally be.
5) The next thing I do is bend *at the waist*. Only bend my waist. Try not to "curl" your back. A straight back creates a great deal of stability in your stance. Also, over time, it will be far less fatiguing. Finally, I feel a straight back makes it easier to keep your head up and face the shot. For me, bending at the waist feels very tight in my front hamstring.
6) I place my bridge hand down on the table along the line of the shot. If I did everything above, I should be able to close my eyes and just swing the cue and make the ball (I can).
This is a proven method. Everyone I've tought it to immediately shoots straighter. The things you have to come up with on your own are the distance you stand from the ball, and how far you step forward with your front foot.
I can explain further if you wish.
Hope this helps,
Kerry