Throw the ball at half of regular speed to your partner when practicing. A really important part of throwing a curveball is ensuring that you get the direction of the spin correct and that you can repeat this at least 80% of the time. As such, throwing at half your regular speed is a good halfway point to get your action started but to minimize the variables you have to battle.[10]
7. Parallel Feet Drill - Works on upper body mechanics. This drill isolates the upper body. The pitchers face each other chest to chest with the feet at shoulder width. The lower body remains stationary. The ball is held in the glove in the "check-your-pulse" position. The torso twists at a 90 degree angle as the ball is pulled down out of the glove, and in a sweeping arc brought to the power position with the hand always on top, and the elbow at least at shoulder level. The delivery is then made and the proper follow through is checked. The glove elbow finishes pointed toward the sky. The drill emphasizes that the pitcher throws with a "proud chest" that remains closed as long as possible.
One of the most commonly used verbal pitching cues is for an instructor  to say “chest to glove”.  The cue is designed to promote proper shoulder and trunk rotation into the release of the pitch.  Many young throwers struggle with maintaining a firm front side throughout the entire throw and when the mind decides to fire the baseball forward, the front side leaks open, and command issues arise.   … continue reading

a slider spins more toward the vertical axis and hence tend to move across the horizontal plane with a slight to moderate downward angle (otherwise it’s a fat pitch, not a slider) L to R from the vantage of the mound for a LHP and R to L for RHP. A great R hand slider will look like a fastball inside to a RH batter and wind up moving away and down from him so the catcher catches it on the low outside corner of the plate and the batter is completely fooled and half way back to the dugout by then after whiffing at it. A curve spins toward the horizontal axis so that the rotation produces downward force (bernoulli) and hence it “breaks” or curves more than “slides”. A great curve will break 12 to 6 or down the face of a clock. A great slider is more 2 to 7 for comparison. Both wickedly effective pitches when thrown in the right count and settings.

A well-timed curveball can be highly beneficial to pitchers, but a curveball is pretty useless if the batter knows it's coming so that he or she has time to adjust to the swing. For that reason, it's important that pitchers not only master the grip and motion of the curveball but also the secrecy of the grip itself, which is necessary for fooling the batter.

Both partners will begin in a standing position, facing forward so athletes are side to side, each with one end of the KB Powerbands. The athlete performing the shoulder prevention exercises will put their hand or a towel high in their armpit, bring their elbow to their side, and make a 90 degree angle at the elbow. Holding the KB Powerbands athletes will maintain this position with their arm and begin to rotate just the bottom portion of their arm (forearm and hand) away from their body. It is important athletes do not let their elbow move away from their body. Keeping the elbow tight will allow athletes to experience the full benefits of the pitching drill. Athletes will move the hand away from their body at a normal speed while taking 4-5 seconds to control the resistance as the hand moves back toward the body.
Any baseball pitch begins with how the pitcher grips the ball. To throw a curveball, a pitcher must hold the baseball between his thumb and his index and middle fingers, with the middle finger resting on the baseball seam. When the pitcher comes through his motion to throw the ball, he snaps his wrist downward as he releases the ball, which gives the ball topspin. If the pitcher throws properly, the back of the his hand will be facing the batter at the end of the motion. The ball will break down and away from a right-handed batter if thrown by a right-handed pitcher.
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.