It is imperative that our goalkeepers do a better job of getting their feet set a fraction of a second before a shot is struck. By set, we mean that the feet should be parallel (as opposed to having one foot ahead of the other), and that the feet are an appropriate width apart (slightly wider than shoulder width for most keepers). In addition, weight should also be evenly distributed--don't lean left or right.
The set position can be achieved with a small hop that scrapes the ground, or with two small chop steps.
The real key here is to get set at the appropriate time. Learn to recognize cues for a shot: forward's head going down, leg going back, plant leg locking. Then, make every effort to get set just before the foot hits the ball (or before a head hits the ball, as the case may be).
Ultimately, if you set too late, the ball will be in the goal before you can react; if you set way too early, you are likely to get "stuck" and be slow to react.
(In reality, as we go forward, "pause" might be a better word to use than "set," as the feet merely remain in this position for a split second, and then hopefully the keeper responds with a quick first step in the direction of the ball).
Whatever the terminology, visualize being in a good set position just as a ball is struck. This is guaranteed to make you a bette goalkeeper.