When a keeper plays the ball, he is active and acting, he makes the save and is able to clear the ball. When the ball plays the keeper, he is acted upon, he is a passenger. Typically in these situations, he makes the save by either getting hit with the ball or desperately lunging to reach it. The clear is secondary, if at all, and the rebounds are dangerous. There's no one perfect ready position for every keeper.
Every keeper is different and as such, has their own strengths.
Having said that, there are common things that keepers are looking to do as they play a shot. They're looking to make the save, first off. They're looking to clear the ball to safety as quickly as possible. In evaluating the ready position of successful keepers, several common characteristics emerge. Starting from bottom to top, the first thing you notice is that the keeper's weight is up on the balls of his feet, about shoulder-width apart.