Create a series of hand signs to signify each pitch. A touch of the cap signals a fast ball. A brush of the sleeve calls for a slider. Patting the chest means a changeup. Pointing to the wrist asks for a curveball. Develop signs that can be remembered easily by you and your catcher. Go over the signs during practices and games until they're memorized.
Tell the catcher before each inning which sign in the series will be the one to relay to the pitcher. Having only one sign gives the opposing team an opportunity to know which pitch is coming. Run through multiple signs, telling your catcher that the second sign you give will be the pitch to throw. Create a "wipeoff" sign that negates previous signs and restarts the series.
Position yourself during the game in a place where the catcher can clearly see you. A dugout has a number of possible obstructions, from fence posts to moving players. The catcher needs to have a clear line of vision to pick up the sign.
Give the proper sign for the pitch you want thrown. If you ever see a pitch thrown that you didn't call for, ask for time and meet with your catcher and pitcher on the mound. Throwing the wrong pitch at the wrong time can increase the opponent's chance of success.