In my opinion, the changeup is the most deadly pitch in the game. For example, look at Johan Santana; he dominates the game with it.
Reason being, he can place it wherever he wants to and can throw it whenever he wants to. He proves you don't have to be overpowering to be successful at any level.
Level of Difficulty
With the most effective pitch in baseball comes some difficult aspects in throwing it. The way that I teach it may be different then you.
The most important thing to realize is that the grip of the changeup is meant for you wrist to be able to break and not to be straight or stiff. The reason it does this is because there is no pressure under the ball.
It's All in the Fingers
The concept then of the changeup is to let the ball come off your fingers. Your wrist then should be in the lead when you begin to release the baseball.
As your hand comes through behind your wrist, the ball is forced away from your body by your fingers.
The key here is to make sure you have a down-and-away motion with your arm. This causes the ball to have a heavy backspin as well as to break away from a RH batter assuming the pitcher is LH.
This backspin on the ball creates a force and slows it down as it cuts through the air. Remember to keep your arm speed the same as your fastball.
You DO NOT want to slow your arm speed down to slow the ball down.
Get a Grip
The grip can be tricky and is important for accuracy and effectiveness. The ball must lie or rest on the balls of your fingers (meaning where your fingers are attached to your palm).
You don't want to have the ball jammed up in your hand because that makes the pitch harder to control. Keeping your pinky finger on the side of the ball helps immensely.
(Kids want to put that finger under the ball; remember we don't want to have pressure under the ball.)
The Feel of It
Always concentrate on letting and feeling the ball come off the fingers in a down-and-away motion.
If you throw the ball with the seams, you can throw it against the seams if you wish by just rotating the ball 90-degrees.