Jinkster,
Jimmy got right to the heart of it and learning to get into that inline, expanded position will greatly increase your ability to execute a smooth, controlled shot. Where you're at now you're pulling the majority of the weight with your arm and shoulder. This works ok when you're fresh and strong but as you begin to fatigue that's when you'll start to notice alot more shaking and the groups really start to inexplicably widen up. When you pull with the back and get into better alignment the draw weight will feel lighter, holding will become easier and the string will get off the fingers easier.
To work on getting into that position you can do a few things. First, when you feel like you've hit full draw try to pull your string arm elbow behind your head. Depending on how much tension you carry in your arms and shoulder, as well as how flexible you are, it may take some time to feel comfortable with this. If you have someone available to help you have them stand behind you and put their hand back and to the left of your elbow while you try to rotate your elbow to touch it. Another thing someone can help with is checking if you are tensing muscles up in your forearm, bicep and shoulder while drawing. If you engage the back and leave the arm to act as a hook the muscles in the arm should have very little tension in them. Have a partner touch your bicep and shoulder at full draw. If it is solid (like you're flexing) you need to work at relaxing the muscles. I've seen a lot of people who could have great alignment but never really learn to relax and let the back become the primary puller.
Much of this is easier to explain in person or visually. Hopefully the above makes sense. I'm betting the video Jimmy is putting together will illustrate exactly what to work on. He's got fantastic form and he's great at explaining things in easy terms.