"The best way to hit the curveball is to hit the fastball" That quote from a major league hitter has a lot of truth in it. Before we talk about hitting the curveball a hitter must understand that his timing and approach are always geared to the fastball. There have been many excellent hitters who don't hit the curveball very well, but there have not been any who cannot hit the fastball.
The three main things a hitter must learn to hit any curveball is to: 1) pick up rotation, 2) learn the hitter's zone for the curveball and 3) hit it with the fastball swing. The best way to get better at 1 and 2 are to stand in on pitchers when they are throwing in the bullpen.
1) When the hitter is standing in during a pitcher's pen he can have his full concentration on just watching the flight of the ball without having to swing. This allows him to begin to learn to pick up the ball early (out of the pitcher's hand) and stay with it long (into the hitting zone). He will also begin to pick up rotation earlier and earlier.
2) He will learn that a curveball that start at the bottom of the hitting zone is not a good pitch to hit, while a curveball that starts at him or above the hitting zone may become a very good pitch to hit.
3) Many young hitters slow down or "feel" with their hands on a curveball. The hitter must let himself take his good swing at the curveball - not change it.