Instructions
Press the "A" button on the Wii remote to begin the serve.
Watch the arc of the ball. It will go up until reaching its highest point and then fall back down. Hit the "A" button to start the serve several times to get the timing down since the ball will simply hit the court and you get to begin again.
Swing the Wii remote forward to strike the ball at its highest point. Begin your swing while the ball is still on its way up. If you time it right, the ball will cross the net much faster with a small trail of smoke behind it.