Until you get your muscle tone built up you are going to shake when you hold you bow very long. However, you should realize that you cannot hold a bow perfectly still. Even when it seems you are locked on a target you are moving slightly. The key to good shooting is to let your pin float around the aiming point until the shot goes off. If you hold long enough to begin to shake, let down and start over. When you shake from holding too long, your holding arm becomes tense and you transfer the tension to the shot when you go ahead and release. Continue to practice and over a period of time, you will notice less and less shake and you will be able to hold longer before fatigue and shaking starts.