If you're looking to improve your pitching speed, there are a few training tips that can guide you in this endeavor. When working on pitch speed gains, bear in mind that the commonly held belief that superior pitching revolves entirely around having a strong arm is actually untrue. Much in the same way that a tennis serve involves the entire body moving in a coordinated sequence of movements, a lightning-fast baseball pitch involves the entire body as well. Remember this when using the tips listed here, and concentrate on training, utilizing and sequencing all the muscles you'll be using in order to put serious force and velocity behind your throw.
The first general but important tip is to keep yourself in shape for pitching by training your core area. Your core area includes the muscles in your pelvis, hips, lower back, and abdomen. Keeping this area in shape is crucial for most athletes to provide balance and stability, and also to provide the bulk of the force behind your pitch.
You can see the core muscles of a pitcher in action when they lift their leg and twist at the waist away from the batter before throwing the ball, twisting back towards the batter to release the ball. This motion is what gives the throw its' thrust, and pitchers with out-of-shape core areas wind up putting too much of the exertion on the shoulder muscles, leading to slower pitches and eventual shoulder injury. Keep your core area fit and strong with exercises like the Russian Twist (while holding a weighted medicine ball), the plank, the barbell squat and back extensions.
Branching off this concept of engaging the core area in your throw, another great tip that will add considerable speed is to practice a long forward stride that is at least 100% of your height upon leaning forward to throw the ball. One way to help you reach this distance is to mark it out in the dirt in front of you with the tip of your shoe, a common practice with most pitchers. In addition to this, you'll want the forward foot of your stride to land flat with the foot pointed right at the target, with your back hip higher than your front hip, and your throwing arm behind you right up until your front leg is planted. Your back leg should straighten and be used to push off before your front leg plants. This is all part of the whole body movement involved with a fast and solid pitch. Just before you fully extend your arm to release the ball, your hips and trunk should have rotated to face the batter.
One good last tip is to practice pitching with weighted baseballs. These exist for the sole purpose of training, and are intended to make the arm more efficient at throwing by getting the muscles in the arm used to a heavier weight object. The arm muscles adapt to the handling of the heavier weight, developing a "muscle memory," which will respond to the pitching of a normal weight baseball with a degree of muscle force that has been trained to exert much more force than necessary. Training with weighted baseballs can bring significant pitch velocity gains in a short period of time, and has the added benefit of developing shoulder muscles to a position of strength that leaves them less prone to injury.