A slider is the third fastest pitch in baseball. (The No. 1 fastest is a four-seam fastball and No. 2 is a two-seam fastball.) It's important for pitchers, parents and coaches to learn a proper slider grip and to learn correct throwing technique of a slider to ensure and promote arm-health. A slider is gripped like a two-seam fastball, but held slightly off-center.
No two pitchers ever throw exactly the same, but their deliveries share three components: balance, timing and power. (See how to work on some of these: The Keys to Improving Pitching Speed and Power.) Every good pitching drill must address at least two of these components. It also must be tailored to your individual style. For example, say you specialize in sidearm pitching. A drill that teaches throwing downhill and forward trunk tilt would be a waste of your time.
I learned this pitching drill from Coach Scott Birchler who was a high school coach at our rival high school but as I got older and made the transition from player to coach, he helped me a lot. This pitching drill that he taught me is great for any pitcher who wants to be more consistent with their curveball or slider and fixes two of the biggest problems in guys who throw these pitches, dropping that elbow and not finishing the pitch.