In this video you will find 3 drills that teach players how to control their serve toss:

1. Lift the Ball with No Spin - by doing that, the player will push the ball up instead of throwing it. This results in more control over the ball.
2. Keep the Arm Extended Towards the Sky - after releasing the ball (above eye level) the player should keep the arm up, extended, until the ball bounces. This will, later on, help with body control during the serve.
3. Let the Ball Bounce in front of the Front Foot - the third drill focuses on the ball placement which should be out in front and, if let bounce, it should land about a foot in front of the front foot.

All the above drills, practiced in order, will allow the player to develop a consistent toss and as a result, the serve will be easier to learn or improve.