The drop ball can be an extremely effective pitch, even if there is not much break to it. It throws the batter off by being slower than the fastball, as well as by dropping off just as he swings.
To grip a drop ball, put your index and middle fingers together, placing them on a single seam. Some pitchers prefer using their index finger to grip the seam, while others prefer using their middle finger. You want to be sure that you hold the ball so that four seams will be used to cut through the air. This will make it drop more than if only two seams cut through the air. The placement of the thumb is different for each pitcher. Experiment to find what placement keeps the ball from leaving your hand too soon or from being thrown straight into the ground. The grip using the middle finger is shown below:
When you throw a drop ball, you want your initial arm motion to be the same as your fastball's, for you don't want the batter to be able to see that your going to throw a curve. This takes a lot of practice, so don't worry too much about that aspect at first. To throw a drop ball, rotate your wrist a quarter turn toward you, at a right angle to the rest of you arm. Whereas for a fast ball, you want your wrist to be loose, when throwing a drop ball, your wrist is fairly locked in place. As you throw it, pull your arm down to give the ball a lot of top spin. Your wrist should not twist as you release the ball. DO NOT TWIST YOUR WRIST TO TRY TO PRODUCE SPIN. (This will result in injury.)
You want to throw your drop ball slower than your fastball so that it has a chance to break. You may also want to throw your drop ball more overhand (though not obviously), depending on whether you want it to break straight down, or at more of an angle. You may also find that shortening your stride a little bit may help. One possible arm motion is shown below:
